A botanist is a person who studies plants in a scientific way. The usual picture is someone out in the wild — hills, forests, wetlands — collecting plants, naming them, and asking how they live where they live. Though actually, a lot of botanists spend their days indoors too: in a lab, a university, a museum, or a seed bank. They may work on crops and garden plants as well as wild ones. The word is a job title, so it feels a little formal; you put it on a CV or say it in a science class. In casual talk someone might just say “a plant scientist,” but botanist is the ordinary professional word. People also stretch it a little for anyone who knows plants unusually well, even if that is not their paid job.