The term 'proficient in' can be described by the crossword solutions 'VERSED', 'UPON', and 'goodat'. 'VERSED' means to have knowledge or experience of something, while 'UPON' implies being knowledgeable about a certain topic. Lastly, 'goodat' suggests having a high level of skill or expertise in a particular area. Therefore, the term 'proficient in' can be defined as having a deep understanding of a subject, being highly skilled in it, and having a great deal of experience with it.