Absolute majority is a supermajoritarian voting requirement which is stricter than a simple majority. It means that more than half of all the members of a group, including those absent and those present but not voting, cast votes in favor of a proposition.
As an example, let's say that a member of a club of 100 members proposes a new bylaw. According to the club's practice, for the bylaw to pass, it requires an absolute majority. The results of the vote are 40 yes votes and 30 no votes. The rest of the voters either abstained or did not vote. Even though this arrangement is a simple majority, since an absolute majority for the club is 51 members, the proposed bylaw fails.
This is rarely used as a voting process, and most voting decisions require a simple majority or even just a plurality.
See also