Dust
Also called: dust output, dust limit, dust threshold, dust UTXO, dust attack, 546 sats.
Dust is a bitcoin output so small that spending it would cost more in fees than it's worth. Nodes don't relay transactions that create it by default.
Dust is an output whose value is too small to be worth spending: the fee to use it as an input would eat most or all of it. Bitcoin Core treats outputs below the dust limit as non-standard, so by default it won't relay transactions that create them.
The dust limit
The dust limit is relay policy, not a consensus rule. Bitcoin Core calculates it from the size of the output plus the input needed to spend it, at a default "dust relay" fee rate of 3 sat/vB. That gives a different threshold for each output type, for example 546 satoshis for P2PKH, 294 for P2WPKH and 330 for P2TR. Nodes using BIP 61 reject messages had a dedicated code, 0x41, for outputs below the dust threshold.
Ephemeral dust and anchors
Some protocols deliberately create tiny outputs, such as anchor outputs used for fee bumping. BIP 433 describes Pay to Anchor (P2A), a keyless output type, and relay policy that allows a single dust output, even one worth zero, as long as a child transaction in the same package spends it. The P2A dust threshold is 240 satoshis.
Dust attacks
Attackers sometimes send dust to many addresses, hoping the owners will later spend it together with their other coins and so link their addresses. Wallets often let users freeze such coins, but frozen dust stays in the UTXO set and can be spent by accident after a restore. BIP 451 proposes a standard way to get rid of it: spend the dust to a single OP_RETURN output containing ash, with the entire value going to the fee, signed ALL|ANYONECANPAY so unrelated disposals can be batched together.