A lifeform was a generic term for a living being.
Strange icarons had no harmful effects on organic lifeforms, but interfered with starship jump engines. (PROSE: GodEngine) Conversely, normal icarons "preferred" to be in hyperspace and caused paranoia and psychotic behavior in carbon-based lifeforms. (PROSE: Original Sin)
Gabrielideans were a liquid-based lifeform. One of their soldiers fighting for the Time Lords in their war against the Enemy was captured by them and turned into an idea-based lifeform known as The Shift. (PROSE: Alien Bodies)
When the Tenth Doctor and Donna Noble visited the Library, its computers confirmed they were the only humanoid life present, but then told them there were over "a million million lifeforms" in the Library. (TV: Silence in the Library)