Definitions are your knowledge base, they ensure that you actually know what these words that you keep using mean.
Without learning definitions can you ever really understand a topic?
Dependent on how you like to learn, the process of learning these definitions may be a welcome break or a slog but it as to be done! All of the sciences have many definitions and unfortunately in the vast majority of cases your wording is critical; miss out a key word and you lose the mark even if the jist of what you have said is identical!
In most cases, learning these definitions will involve simply memorising them. Every year I have students who seem to think memorising is a cop out and not proper learning but as the definitions are the basis of the subjects you have to start by learning these foundations inside out.
Here are my tips:
Use mark schemes - as the definitions so often have to be word perfect, look through past papers and mark schemes and base your definitions on those given by the mark schemes. Far too often I have seen text book definitions be rejected by mark schemes - even those very textbooks endorsed by the exam boards.
Make a list - You don't want to be flipping back and forth between your notes to find your definitions - this is a massive waste of your value time. Dedicate an hour or so to writing up all of your definitions onto a sheet or too of paper and keep it at the front of the folder so that it is easy to find.
Make it interactive - Memorising can be dull and you may find that you quickly switch from the task at hand. Try to involve a friend or a family member. Give them your definition sheet and get them to ask you for the definition of each word - turning the task into a quiz. As your definitions list will have all of the answers you don't need to find someone that know the topic - anyone can help! You could even set prizes for getting them all correct!
Revisit them often - Once you can recite all of the defintions in one sitting flawlessly you may think that you are done. However, I cannot emphasise enough - don't stop here! Just because it is in your short term memory does not mean it is in your long term memory and you don't want all of your hard work to be worthless. Revisit the definitions weekly, even just for one quick quiz; this will ensure that you don't forget them.
Visit the resources pages - I aim to eventually get all of the definitions sheets on the resources pages. Keep checking for your subject and exam board!