Zoology is the branch of science that deals with animals — their bodies, their behaviour, how they grow, how they are grouped into families, and how they live with each other and with their surroundings. It sits inside biology, the study of all living things, as the animal half; botany, the study of plants, is its partner.

You meet the word in two ways. First as a subject you can study: 'She's doing zoology at university', 'a zoology degree', 'the zoology department'. Second as a body of knowledge: 'what zoology tells us about memory in birds'. It is a formal, academic word — in ordinary talk people say 'animal science' or just 'studying animals'. Worth knowing that in real universities the name is fading: many departments have been renamed 'biological sciences' or 'ecology and evolution', because modern research crosses the old animal/plant line. So a student today may study exactly what zoology means without the word ever appearing on their certificate.