Egress Cost
also called Data Transfer Out
The charge for moving data out of a cloud provider or across its zones and regions — usually the least-anticipated line on a cloud bill.
Data in is typically free; data out is not, and the asymmetry is deliberate. The rates that catch people are not internet egress, which is at least visible, but inter-zone and inter-region transfer, which is invisible in a diagram and can be generated in enormous quantities by a chatty architecture that happens to span zones.
Architectural consequences worth designing for: keep chatty communication zone-local; put a CDN in front of anything served repeatedly; process data where it lives rather than moving it to compute; and check whether a private link or a provider peering arrangement is cheaper than the default path.
It is also the sharpest form of lock-in. Multi-cloud designs that move data between providers regularly are usually paying more in egress than they save in negotiating leverage.