A change (such as a transaction) is ParetoEfficient if at least one person is better off, and if no person is worse off; this is known as a ParetoImprovement.
If a situation allows no ParetoEfficient change, it is said to be ParetoOptimal.
A transaction is ParetoEfficient if the gainers' gain is at least enough to compensate the losers for their loss, and the losers in fact receive the compensation.
If a transaction would be ParetoEfficient via compensation, but the compensation doesn't occur, then it's KaldorHicksEfficient.