Reading the Investopedia definition of outstanding shares, it seems like it includes all cases for a share. So how is it different than a regular share? In other words, why not just call it a share?
outstanding shares are the shares(regular shares) that are still tradable in the market, where the firm in question is listed. The term is primarily used to distinguish from shares held in treasury(treasury stock), which have been bought back(buybacks) from the market and aren't currently tradable in the market.