1 [concept]
Dust is an amount of a crypto asset which is below the minimal miner fee and therefore unmoveable. Typically this is in the value range of fractions of a cent, but when miner fees are high dust can include amounts into the dollar range.

1.1 Example

When moving coins off of an exchange, there will sometimes be tiny amounts left over worth fractions of a fraction of a cent. When this amount is so low that the cost of miner's fees to move it is higher than its value, it is known as dust