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Half-life: The amount of time it takes for half of the atoms in radioactive material to decay.

Table can be found at Common Radioactive Isotopes Produced During Nuclear Reactions

Haploid: A single set of chromosomes (half of the full compliment of a genome) which is found in the sex cells of sexually reproducing organisms.

Herbaceous: Term referring to any plant that does not have a woody stem.

Hermaphrodite: Having both male and female reproductive organs.

Heterosporous: A plant that produces both male and female spores, but does so separately.

Heterotroph: Literally means "other feeder." A term used to describe any organism that does not produce its own nutrients.

Hindgut: The portion of a vertebrate digestive tract that contains the colon and the rectum.

Homologous: Any anatomical structure or behavior that is shared by separate species because of common ancestry. Although an anatomical structure may be shared, it can have a different function. An example would be pentadactyly in terrestrial animals.

Hormone: A biochemical substance formed by one or more cells that can stimulate or inhibit other cells.

Hybrid: The offspring between two separate species or subspecies.

Hydrocarbon: A molecule that contains only hydrogen and carbon.

Hydrolysis: A chemical reaction where water is used to break down a compound.

Hydroxyl group: A molecule comprised of one hydrogen atom to one oxygen atom, and has a free bond that can attach to another molecule. In chemical symbols, a hydroxyl group looks like ~OH. An organic compound that has a collection of hydroxyl groups is called alcohol.

Hymenium: In fungi, regularly arranged spore-producing bodies.

Hypha: The filament-like structures in multicellular fungi that form the mycelium. These structures promote nutrient distribution. Plural hyphae.