Habitat fragmentation describes the breakup (fragmentation) in an organism's preferred environment (habitat), causing population fragmentation and ecosystem decay.

Heathland is a shrubland habitat that is characterised by the presence of heather.

Hectare (ha) is a unit of land equal to 10,000 square meters or 2.4711 acres.

Broadly speaking, heterodox economics refers to economic theories and methodologies that differ from or directly contradict mainstream neo-classical economics. Heterodox economic approaches are used by ecological economists who seek a move away from policies based on conventional environmental economic thinking.

The Holocene is the current geological period and started about 11,700 years ago after a process of glacial retreat and a mass extinction of megafauna such as the sabre-toothed tiger, the mammoth and the woolly rhinoceros. The start of the Holocene roughly corresponds with the invention of agriculture and the start of a process of rapid growth and spread of the human species worldwide.

Holocene rewilding is a rewilding strategy that takes the early to late Holocene (after the emergence of agriculture and before the industrial revolution) as its historical baseline for ecosystems. Whereas Pleistocene rewilding is associated primarily with the reintroduction of large carnivores, Holocene rewilding emphasises the role of large herbivores such as (ancient) cattle and horse species in managing the landscape. Holocene rewilding revolves around the wood-pasture hypothesis by the Dutch conservation biologist Frans Vera, which states that after the last ice age, European lowlands will have developed into semi-open pastures dotted with large solitary trees and small patches of trees and shrubs, kept open by large herbivores. This hypothesis is contested.

Hydrogenation is a chemical reaction between a hydrogen molecule (H₂) and another molecule or element in the presence of a catalyst such as palladium, platinum or nickel. In the food industry, hydrogenation is often used to turn liquid oils into solid fats. The partial hydrogenation of oils is used to produces trans-fats. Hydrogenated fats and trans-fats are contested for their health impacts.

Hydroxyl (OH) radicals are highly reactive molecules responsible for the initial reaction leading to most methane destruction in the atmosphere, and also important for the removal of many other atmospheric pollutants. Radicals are molecules or atoms with an unpaired electron, often making them very reactive.