A collectible that is undervalued relative to its long-term potential — typically because it has not yet been recognized by the broader market. Sleepers are the holy grail of bargain-hunting collectors.

Examples have included pre-1980 video games before WATA grading, early Pokémon cards before the 2020 surge, and pre-2010 modular Lego sets. Identifying sleepers requires deep category knowledge and patience for the market to catch up.