Acidification refers to changes to the chemistry of a body of water that make it more acidic over time (i.e. increased hydrogen ion availability). This change in acidity can then affect many other reactions that take place in water, including those important for ecosystem functioning.

Aeration is the process by which air is circulated through, mixed with, or disolved in a liquid or substance.

Agri-food system refers to the constellation of social, political, economic and environmental processes and actors involved in the production, distribution and consumption of food and related agricultural products. It encompasses all varieties of food production including aquaculture, forest-based production, livestock and crops, and focuses on both their biophysical and socio-economic characteristics.

Agricultural intensification is the process of increasing the inputs of agricultural resources (e.g. seeds, labour, fertilisers, pesticides, technologies, knowledge) to increase the level of yield per unit of farmland or pasture. Agricultural intensification is not always clearly or consistently defined and is often confused with the term intensive agriculture. Unlike intensive agriculture, which could be seen as a specific system of agronomy, agricultural intensification is a general process that can apply, in principle, to any type of agricultural production. Examples of agricultural intensification may range from using new pesticides in intensive agriculture to intensifying the use of indigenous and context-specific knowledge in local farming practices. Although agricultural intensification can take many forms, it always involves the intensification of some types of agricultural input with a view to increase levels of yields.

A form of crop farming that is based on the growing of a single crop type on a field at a given point in time. Agricultural monocultures sometimes follow a rotational pattern where different crop types such as maize, wheat or soybeans are grown successively on the same field. The use of agricultural monocultures is typically based on the principle of economies of scale. The principle here is that the costs of inputs such as machinery, labour, fuel, herbicides, fertilisers and land per unit of output (kg yield) can be kept relatively low if the diversity of crops that are grown in an agricultural landscape is minimised. Agricultural monocultures are controversial within the environmental movement. Amongst other things their efficiency is debated and critics point out that the practice of agricultural monoculture can lead to externalities (i.e. costs such as biodiversity loss, water pollution or a lack of resiliency that are not reflected in the final cost of the product) and also for the most part  goes hand-in-hand with ongoing corporate consolidation in the food system.

Agricultural production is the range of practices and approaches that are employed to transform agricultural inputs (e.g. labour, knowledge, land, water, seeds, fertilisers, pesticides) into agricultural outputs (e.g. yields and environmental impacts). Different types of agricultural production include precision farming, agroecology, organic farming, and intensive livestock farming.

Agricultural productivity is the ratio of yields relative to agricultural inputs (e.g. labour, knowledge, land, water, seeds, fertilisers, pesticides).

Agricultural yield is the average net output of agricultural product (e.g. in kCal, grams protein, or net profit) per unit of farmland per year. The total amount of farmland includes all land that is required to generate the output (e.g. also land that is used to grow feeds or to produce manure).

Agrochemicals are chemicals, such as a pesticide or fertiliser that are used in agriculture.

Agroecology is commonly understood as a science, a practice and a movement. As a science, it uses principles from the field of ecology to study the interactions between organisms in agroecosystems. It is often associated with transdisciplinary and action-oriented research, and the study of the entire food system. As a practice, agroecology combines indigenous and traditional knowledge, and scientific research, to generate productive, sustainable and resilient farming systems with minimal external inputs. This is achieved by optimizing processes and interactions occurring within agroecosystems, for example through crop rotations, cover crops, polycultures, crop-livestock integration, agroforestry and minimal tillage. It is generally associated with smallholder farming, and focuses on the production of nutritious food suitable for personal consumption and local markets. As a movement, agroecology seeks to address power imbalances within the food system, and generate a more just and equitable food system based on the principles of food sovereignty.

Agronomy is a science and practice seeking to understand and improve the cultivation of plants for food, fibre and fuel. Agronomists focus on a variety of factors relating to crop production, including yield, disease, climate and soil.

Alternative food movements generally exist to challenge the current negative consequences of food systems. Fair trade, organic, food justice, food sovereignty, vegan and vegetarian movements can all be seen as offering alternatives to the status quo and as such form part of the "alternative food movement;" it can be said that there are several movements, rather than just one.

'Alternative protein' is a phrase used to describe either specific protein sources (such as microbial mycoprotein) high-protein foods which can be considered alternatives for animal proteins and animal source foods. The term is most typically used of proposed novel foods, but in the context of discussions of radical changes to the food system to achieve greater sustainability is also used of traditional foods whose consumption could be increased. Some foods commonly referred to by the term alternative proteins are insects, ultra-processed plant-based meat substitutes, cultured meat, single-cell proteins, and minimally processed legumes.

Anaerobic processes occur in the absence of oxygen. For example anaerobic respiration occurs when oxygen is not present.

Animal ethics attempts to use moral reasoning to move beyond existing norms (e.g. regulations that may have been influenced by stakeholders with differing interests) and propose directions of change based on specific ethical theories. The two dominant families of ethical theories around human relationships with other animals are rights theory and utilitarianism.

Animal welfare science aims to objectively assess good or bad animal welfare in order to inform interventions, often focusing on win-wins that improve both human and animal lives. Based on scientific study many measures of welfare have been proposed, such as frequency of injury, ‘stress hormone’ (cortisol, corticosterone) levels in the blood, rates of negative behaviours like stereotypes (repeated, non-functional, often harmful behaviours) and conflict, or the occurrence of positive behaviours like foraging and grooming. These measures are used in a variety of ways: they inform new animal welfare laws; they feed into further scientific study; and they are used as indicators of good and bad welfare in assessment frameworks, for use in monitoring, comparison and marketing.

The Anthropocene is the proposed (and, so far, unofficial) name for a new and current geological epoch distinguished by humanity’s significant impacts on the planet’s physical, chemical and biological systems, including climate and ecosystems. The exact start date and definition of the Anthropocene remain debated.

Anthropocentrism literally means human-centred. It refers to a philosophy and worldview that bases moral worth on the capacity for analytic thought and judgement, and therefore sees humans as separate from and distinctive to the rest of the natural world. In some cases, this thinking is associated with the perception that nature only has value in the extent to which it can be exploited to meet human needs. It is often suggested that capitalism and western liberal democracy are informed by an anthropocentric worldview, and some people blame anthropocentrism for climate and environmental crises and the depletion of natural resources.

Aquaculture refers to the breeding, rearing and harvesting of animals and plants in aquatic environments.

An aquifer is an underground layer of rock, sand, gravel or earth that contains water or allows water to pass through it. Aquifers normally contain fresh water and are in many regions an important source of water for drinking, agriculture and other human activities.

Arable crops are those such as wheat and barley, which require good soil quality and a favourable climate to grow, and land amenable to the use of ploughing and harvesting machinery. Arable land is by definition land used to grow arable crops, in contrast to land used for fruit and vegetable crops and for pasture used to feed grazing animals.

Arthropods are invertebrate animals such as a spiders, shrimp or beetles that have an exoskeleton, a segmented body and paired jointed appendages (paired wings, legs or antennae).