A quorum is not necessarily a majority of members of a group, but the minimum needed in order to conduct business. For example, if two members of a group are absent, there can still be a quorum, meaning the meeting can go on without them.
The noun quorum is plural of qui in Latin, meaning "of whom." The first quorum was an eminent group of justices of the peace. The word quorum was used in the commission papers that gave them the authority to act. Over time the current meaning, the minimum number needed to conduct business, was added, but the word still retains the meaning of a select group, as well.