There are terms that can be defined easily, such as glass or table.
Don’t forget to support your arguments with evidences and references.
Conclude your essay by restating the main points and the thesis statement.
There are millions of topics to choose from, and if you are not sure about which one you should pick, find a definition essay topics list and choose something you like from it.
Just don’t pick something too concrete, like “tree” or “shoe”, because there won’t be much to talk about.