Five definitions: ozone hole, mass extinction, non-biodegradable waste, smog, grazing food chain

Five of the six are defined, as the question allows.

(i) The ozone hole

A region of the stratosphere, seen first and most sharply over Antarctica, where the ozone layer has been thinned far below normal by pollutants such as CFCs and halons; the thickness of the layer is measured in Dobson units.

(ii) Mass extinction

The loss of a very large share of the earth's species within a geologically short period. The fossil record shows five such episodes in the history of life; the sixth, now under way, differs because it is caused by human activity and is far faster.

(iii) Non-biodegradable waste

Waste that micro-organisms cannot break down into simpler stable compounds, such as plastic, glass and metal.

(v) Smog

A mixture of smoke and fog that hangs over a city, made worse when pollutants from vehicles and industry are trapped near the ground.

(vi) The grazing food chain

A food chain that begins with green plants and passes energy to the herbivores that eat them and then to carnivores, for example grass, then goat, then man.