Rainbows and Steelhead have the same Latin name and are, biologically, the same fish which can interbreed and yield fertile offspring. All trouts and salmons owe their strain diversity to the ice ages during which populations became isolated behind of or in front of ice dams in their native range spanning the upper Pacific rim. Some became non migratory river dwellers, others fed in lakes and spawned in streams yet more roamed the Pacific before ascending their natal rivers to reproduce. And every conceivable variation in between (plus Cutthroat). Some coastal accessible rivers, the Deschutes is a great example, have/had multiple rainbow strains including none migratory "Redsides", Winter and Summer steelhead that did not interbreed due to staggered spawning time developed via evolutionary species diversity. The Columbia basin, pre-damming and Euroman intervention, had hundreds if not thousands of distinct rainbow/steelhead strains specialized to their home rivers as close as the Sandy and as far as Idaho's Clearwater. Montana's Kootenai River is the ONLY river in Montana with a native strain of rainbow and that is far from the mouth of the great Columbia.
Rainbows are perhaps the most interbred and domesticated of all Salmonidae, manipulated through selective breeding by fish culturists to be many different fish. Our Great Lakes Steelhead mentioned in an above post are from a couple, dominantly Washougal strain domesticated steelhead stocked first in Michigan successfully then from Michigan to New York's Lake Ontario. The presumably non-migratory rainbow stocked in the Catskill's Esopus Cr. surprised fisheries officials by being lake migratory and growing to only 11'" or so before dropping down into the Reservoir to grow to lager size. every rare once in a while, an autumn angler surprises himself with an outsize rainbow eating his streamer in the creek.
So, it would appear difficult to draw a distinct line between Rainbows and Steelhead and many if not most of us are primarily fishing for domesticated though sometimes wild reproducing strains of these fish. With the exception of the Kootenai and its Yak tributary, all rainbows I have caught, and many are fabulous in size and fight, in my beloved Montana are "garbage' domesticated hatcher originated strains quite different from original, no longer in existence in a pure form (except in Argentine Patagonia) non-migratory, late sexually maturing, long lived, McCloud River Rainbows. My west coast fishing partner fishes the Deschutes often and bemoans the plethora of hatchery strain steelhead ascending his home river while overjoyed on the rare occasion he catches a real native fish. NY/PAs upper Delaware enjoys hosting an early (1880s) if not actually pure population of California non-migratory rainbows and we are lucky to have them.
Suffice it to say, as an angler not an ichthyologist, I know the difference between a Rainbow and a Steelhead...when I hook one.