Zoological is the adjective for zoology, the scientific study of animals — how their bodies work, how they behave, where they live, how they are related to one another. So anything described as zoological belongs to that science: a zoological study, a zoological survey of the island, the Zoological Society of London.

Its most famous use is in the phrase 'zoological garden', which is the full, old-fashioned name for a zoo. That is literally where the word 'zoo' came from — people got tired of saying the long version and cut it down. Today 'zoological garden' survives mostly in the official names of older zoos and on formal signs; nobody says it in ordinary speech.

This is a formal, academic word. It lives in textbooks, museum labels, research papers and institution names. In daily English you would say 'about animals' instead. Also keep in mind that 'zoological' does not mean 'animal-like' or 'wild' — it means 'belonging to the science of animals'. A zoological drawing is a careful scientific illustration, not a picture that looks like an animal.

Pronunciation catches learners out: it is zo-o-LOJ-i-cal, four syllables before the ending, with the two o's at the start pronounced separately — not like the word 'zoo'.