One is to essentially give meaning in other terms to a word or concept which is not understood by someone. This is what it means to define something, to put the meaning of one word into other words that a person can understand more clearly. The other meaning of the term is even harder to describe. To give something definition is basically to make it stand out more clearly, and this is an important aspect of the word, especially when it comes to dreams.
If you have a dream about something that needs to define something else or where you feel like you need to define yourself then this is because you feel the same way in real life subconsciously. This usually happens when a person feels like they are not as unique as they could be. In reality this does not mean you are not unique, it just means that you aren’t doing your best to show who you really are to others. Sometimes this is natural because you feel a little shy around others, a little too shy to express yourself.
If you have a dream that you need to define one of your possessions then this suggests that you have a desire to receive validation for your acts and what you’re doing. While this is not uncommon, it is not exactly healthy either and you probably shouldn’t be thinking about how badly you want others to validate your existence and your thoughts. It would be best for you to be more concerned about your own achievements than what other people think about your achievements.