A cleric is a member of the clergy of a religion, especially one that has trained or ordained priests, preachers, or other religious professionals. Its non-specific nature means it is often used to refer to the religious leadership in Islam, where "priest" is not accurate and where terms such as "Alim" are not widely understood in the English-speaking world.
To use cleric is also appropriate for minor clergy who are tonsured in order not to trivialise orders such as those of Reader in the Eastern Church, or for those who are tonsured yet have no minor or major orders. It is in this sense that the word entered the Arabic language, most commonly in Lebanon from the French, as "kleriki" (or, alternately, "cleriki") meaning "seminarian." This is all in keeping with Eastern Catholic and Eastern Orthodox concepts of clergy, which still include those who have not yet received, or do not plan to receive, the diaconate.
The term "clerk" derives from "cleric," since in medieval times the clergy were one of the few groups who could read, and therefore were often employed to do bookkeeping and similar work. The term "clerical work" continues to this day to refer to such functions.