The study was based on an assumption that the concept of function is a very important basic mathematics concept weaknesses of studens teachers and teacher candidates in understanding the concept of function and the role of definition in the beginning stage of mathematics learning. The difficuties in understanding definition using their own words or using language that are relevant to the learning materials difficulties in identifying examples or nonexamples of the concepts the inability to identify and to relate parts of the explanation to the main idea in the definition and the inability to compare the two different definition of a concept. The study indicated that the factors that cause them to have these difficulties are insuffiacient understanding on mathematical terminologies