A bioform is a term for any living creature. The term is heavily used throughout lore in the Metroid Prime series, and has never been used in any of the 2D games. The term has also been applied to individual characters, such as Samus Aran, who is referred to as "Bioform Samus Aran" in at least two Space Pirate communiques.
The vast majority of bioforms encountered in the Metroid series are hostile for a number of reasons. This includes telepathic control (such as by Mother Brain, at least in the original Metroid), a malevolent nature (such as the Ing), or the bioform is merely defending itself. Some bioforms, particularly in the Prime series and Metroid: Other M have been cybernetically enhanced.
In Metroid Prime 2: Echoes, the term plantform is used specifically to refer to plants, such as the Shredder, durable plantform, and Seedburster, and is the namesake of a Logbook category. It is also used in Metroid Prime 3: Corruption in a scan for the Cyrlic tree.
Metroid Prime: Federation Force also uses an alternative name for creatures, xenoform, specifically in regards to the inhabitants of Excelcion. This may be due to these invasive species (originally native to the polar regions) immigrating to the rest of the planet (including Galactic Federation outposts), since the "xeno-" prefix refers to something that is foreign.