Make an addition (to); join or combine or unite with others; increase the quality, quantity, size or scope of WordNet 3.0 & Open English WordNet

To join or unite, as one thing to another, or as several particulars, so as to increase the number, augment the quantity, enlarge the magnitude, or so as to form into one aggregate; to sum up; to put together mentally, as, to add numbers; to add up a column. OmegaWiki

To join or unite (e.g. one thing to another, or as several particulars) so as to increase the number, augment the quantity, or enlarge the magnitude, or so as to form into one aggregate. Wiktionary