Copyright Office has stated, protection is not granted for "ideas, procedures, methods, systems, processes, concepts, principles, discoveries, or devices (as distinguished from a description, explanation, or illustration)."
Definitions. A definition is a statement of the meanings of words or phrases. As a general rule, if there is a limited number of ways to express a definition, copyright law tolerates copying. Short definitions may fall into this category. If not, a few changes in wording should suffice. However, be careful when you take too many word-for-word explanations from one source. The source may complain that you are infringing original expression (that is, they may argue that there are many ways to define the term) or you may be accused of "compilation" infringement.