Cryptocurrency dust are small, fractional values of cryptocurrency units, usually they are impossible to transact or trade because the transaction fees are higher than the available amount. Being below the minimum trading limit, they usually stand idle in user wallets and can’t be used until the wallet is replenished. In the past, dust wasn’t necessarily problem for bitcoin users. The story changed, however, as fees grew higher than ever late 2017, making smaller value transactions much more expensive to send.
The term applies to both cryptocurrency wallets, and cryptocurrency services such as exchanges.
In addition, it is often a tool for certain types of attacks. In order to track additional transactions, large quantities of dust are sprinkled throughout the network. Doing so will effectively target a large number of addresses. It is the attacker’s hope that the tiny amount of funds mixes together with an ‘unspent transaction output’ (UTXO). This way, when it is spent as an input in a new transaction, it can be trackable.