Herbivory is the process by which herbivores eat plants.

1. Thorns, barbs, or spikes
2. Fibrous and inedible tissue
3. Stinging hairs
4. Hairy leaves

Alkaloids are a large group of bitter-tasting nitrogenous compounds found in many plants.

Some examples include caffeine, nicotine, morphine, and cocaine.

A pheromone is a chemical made by an organism that affects the social behaviour of other members of the same species.

VOCs are chemicals produced by plants that act like pheromones between themselves and organisms of other species, usually as a chemical defence against herbivory.

For instance, when cabbages are attacked by specific caterpillars they release a VOC that attracts a specific parasitic wasp that kills those caterpillars.

Folding leaves quickly when a plant is touched may frighten off larger herbivores, make the plant appear smaller and less appetising, and dislodge small insects that have landed on the leaves.

Mimosa pudica is an example of a plant that folds its leaves when touched. 

Many alkaloids act as drugs and affect the metabolism of animals, discouraging some herbivores from consuming the plant that produces them.

Some alkaloids are toxic or poisonous to animals, discouraging some herbivores from consuming the plant or even killing them.