Dust Limit

A dust limit is the smallest value an output may carry before the network's relay rules treat it as uneconomic and decline to pass it on. The reasoning is that such an output would cost more in fees to spend than it is worth, so permitting unlimited tiny outputs would let anyone inflate the set of unspent outputs that every node has to keep. What a wallet user meets is a small balance that will not move: a send is rejected, or the wallet shows an amount it refuses to spend, because once the fee is covered there is nothing left to deliver. This is a policy of node software and of the wallet rather than a consensus rule, so the threshold differs between implementations and shifts with the cost of spending rather than being a fixed number. It is a separate idea from a dusting attack, which is about tracing: dust sent for that purpose is deliberately set just above whatever the relay minimum happens to be.