An abstract mark is a geometric or non-representational shape that doesn't depict a literal object — unlike a pictorial mark, it has no obvious real-world meaning on its own.
Abstract marks are flexible and often age well, precisely because they aren't tied to a specific trend or literal image that can feel dated. The trade-off is that they rely entirely on consistent, repeated use to build meaning — an abstract shape means nothing to a new audience until the brand has put in the work to associate it with something.
They tend to suit businesses with a strong marketing engine behind them, since building that association takes sustained visibility.