The steelhead life cycle begins in a stream, but the fish may grow in the ocean or a large lake before returning to spawn. That difference separates sea-run from lake-run steelhead, even though both are rainbow trout.
Essential Concepts
Sea-run: ocean. Lake-run: large lake. Both spawn in streams.
Related posts: /rainbow-trout-vs-steelhead/ and /steelhead-spawning/