A Member of Parliament is, aptly enough, a member of a parliamentary system, standing as a representative for the voters who elected that individual to the position of Member of Parliament. The term “Member of Parliament” is often used specifically to refer to those individuals who are voted into office within the lower house of the Parliament in question. This is often because the lower house of the Parliament is often the house which actually holds significant power within the parliamentary system in question.
For example, in the Parliament of the United Kingdom, the House of Commons, the members of which are elected by the populace of the United Kingdom, as opposed to being appointed in some other fashion as are the members of the House of Lords, holds most of the power within the Parliamentary system of the United Kingdom. A Member of Parliament in the United Kingdom system, then, would refer specifically to an individual who is a member of the House of Commons. This is often also a side effect of the upper house of Parliament granting upon its members titles which are different than Member of Parliament. Member of Parliament is often abbreviated to “MP”.
A Member of Parliament will most often belong to a particular parliamentary party, made up of other members of Parliament who are from the same political party. Thus, a Member of Parliament will most often vote along the lines of this overall party.