- Iceland
- ichthyology
- illicium
- imbricate
- immature
- incisiform teeth
- incisor
- incisus
- inferior
- inferoposterior
- infra (as a prefix)
- infracaudal
- infraspecific
- ingested
- iniomi
- inter (as a prefix)
- interbranchial
- interbreeding
- interdorsal
- interhaemal
- internasal
- interocular
- interorbital
- interpelvic
- interradial membrane
- interspace
- intersperse
- intertidal
- intestine
- intraspecific
- intricarius
- intromittant organ
- inversion (temperature)
- invertebrates
- iridiscent
- iris
- ISH
- ISNB
- isohaline
- isopoda
- isopropanol
- isotherm
- isthmus
- ISTPM