A dust transaction is typically defined by its value being so low that it is close to or even less than the transaction fee required to process it.
The exact threshold for what constitutes "dust" can vary depending on the cryptocurrency and its network conditions. For example, in Bitcoin, dust is defined as any output that's less than the smallest denomination that can be processed by the network, which can change based on the current transaction fee market.
Dust transactions are considered inefficient because they consume network resources while offering little to no real value transfer. They can clutter the blockchain with unnecessary data, increasing database size and potentially slowing down network processing speeds.
The core issue with dust transactions is that the cost of processing them (i.e., the transaction fee) can be equal to or greater than the value of the transaction itself. This makes them economically irrational, as the sender would essentially be paying more to send the transaction than the transaction is worth.