It stands for expected credit loss; the long-term average cost that arises from a series of defaults expected to take place in a swap (or broadly a debt instrument). The cost associated with such expected credit losses is assumed to be recovered from swap spreads / CDS spreads (or in the case of debt instruments, from any interest rate premiums).

Swap spreads can be perceived as the compensation for the extra credit risk in swaps relative to risk-free instruments.