Define means to state the exact meaning of a word or an object or a thought. It is to give the nature of the basic qualities of something and to be specific in this description. Define is also to give meaning to something, for example, “Courage is defined by selflessness”. It can also mean to describe or distinguish the boundaries of something or to specify or fix with precision. To make something more clear defines it and makes it unique and separable.
To define something means to explain what it is. This is a bit different from describing something: If you describe something you say what it looks like etc, but if you define it you explain its essence. For example, you could define a house as a building with walls and a roof, made to shelter humans, but to describe a house you would talk about what it looked like, how big it is etc.
Define is give a definition for the meaning of a word.