An absolute majority in an election or an election round, means more than 50% of the valid ballots. It is distinct from a relative majority or plurality, terms which refer to the largest group among several, even if that group is 50% or less of the whole.
For these purposes, a ballot counts either as valid or invalid. Therefore, in a system like IRV with multiple counting rounds, a ballot cannot be valid in one round and invalid in a later counting rounds. Thus IRV, although it does guarantee that the eventual winner has an absolute majority of the remaining ballots, does not guarantee that the winner has an absolute majority overall. The only way IRV can meet the guaranteed majority criterion is if any ballot which is not fully ranked is discarded as invalid.