What is dust in Bitcoin?

Spending an output adds data and therefore fees. People often call an output dust when the extra fee to spend it would eat up its value. That cost changes with the fee rate and the output’s spending rules.

Bitcoin Core’s policy uses a dust threshold based on output type and a configured dust relay fee rate. It is not the current market fee rate. Outputs above the threshold can still be uneconomical to spend. Relay policy generally discourages new dust outputs, with limited exceptions; it is not a consensus ban.

A dusting attack sends dust to many addresses, hoping to link their owners by watching how the dust is later spent. Receiving dust gives the sender no access to your coins.