Network traffic leaving a network boundary. In cloud billing, it refers specifically to data leaving the cloud provider's network (often expensive) vs ingress (free in most clouds).
Cloud providers charge for egress (outbound bandwidth) because that's where they have pricing power. Getting data in is free, and getting it out is profitable. Egress costs are often the most surprising cloud bill: a multi-cloud setup that moves data between AWS and GCP can rack up substantial egress fees. Cost-conscious architectures minimize cross-region and cross-cloud egress, cache aggressively at edge layers (Cloudflare, CloudFront), and consolidate workloads to reduce inter-region traffic.
Discovering that a cross-region database replica was driving $30K/month in unexpected egress charges. Moving the replica to the same region cut the bill 95%.
Egress is the most-overlooked cloud cost driver, and architectures designed without egress awareness routinely produce 'why is our cloud bill so high' surprises.
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