Dust
A crypto balance too small to be worth moving because the network fee would cost more than the coins themselves.
Dust is a balance so small that spending it makes no sense: the network fee to move it would cost more than it is worth. A few hundred satoshis stranded in an old bitcoin wallet, or a few pence of a token left behind after a swap, are dust. Wallets accumulate it the way coat pockets accumulate coppers.
Dust is mostly harmless clutter. Some exchanges offer a periodic convert-small-balances feature that sweeps it into something usable; on-chain, it often just sits there, and the honest advice is to ignore it.
The word also names a mild attack. In a dusting attack, someone sends tiny amounts to many addresses, then watches how the dust moves to link addresses together, or uses an accompanying token as bait for a phishing site. Unexpected tokens appearing in your wallet are the same genre: interacting with them, or trying to sell them through the site they advertise, is how the trap springs. Coins you did not ask for cost nothing while they sit untouched, and that is exactly how to treat them.
One design note explains the rest: fees price by transaction, not by amount, so the economics of tiny balances are bad on every chain where blockspace costs real money. Dust is what that arithmetic leaves behind.