Dust.
A UTxO so small it costs more in fees to spend than it is worth.
Pocket lint. Coins so small it would cost more in fees to pick them up than they're worth, so they just sit there.
In depth
Dust UTxOs accumulate when coin selection strategies leave behind tiny outputs. They fragment the wallet, hurt privacy, and may be uneconomic to ever spend on their own. On Cardano every UTxO must satisfy the protocol's minUTxO rule (typically around 1 ada for a plain-ada output, more for outputs carrying native assets, the exact amount depends on the serialized size and `coinsPerUtxoByte`), so "dust" here means outputs that sit close to that floor rather than fractions of an ada.
Example
A 1 ada UTxO is effectively dust if it would cost roughly 0.18 ada to include in a transaction and leaves no useful change behind.