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