Under- is a prefix that attaches to the front of words to create new meanings, usually suggesting something is below, beneath, or not enough.

When under- means below or beneath, it works like in underground (below the ground) or underwater (beneath the water's surface). An underpass is a road that goes under another road, and the underside of a table is the bottom part you don't normally see.

When under- means not enough or insufficiently, it creates words like undercooked (not cooked enough), underprepared (not prepared enough), or underfed (not getting enough food). If you underestimate a puzzle's difficulty, you think it's easier than it really is.

Sometimes under- suggests something hidden or secret, like undercover police officers who disguise themselves to investigate crimes, or underground groups that operate secretly.

The prefix shows up everywhere: in underwear (clothes worn under other clothes), undergraduate (a student studying for a first-level college degree), and underdog (a competitor expected to lose but who might surprise everyone). Understanding this prefix helps you figure out hundreds of words you'll encounter in reading and conversation.