Under the Whyte notation for the classification of steam locomotives, 2-8-0 represents the wheel arrangement of two leading wheels on one axle (usually in a leading truck), eight powered and coupled driving wheels on four axles, and no trailing wheels. In the US, this wheel arrangement is commonly called a Consolidation.

Other equivalent classifications are:
UIC classification: 1D (also known as German classification and Italian classification)
French classification: 140 (also known as Spanish classification)
Turkish classification: 45
Swiss classification: 4/5
Russian classification: 1-4-0

Of all the locomotive types that were created and experimented with in the early part of the 19th century, the 2-8-0 was a relative latecomer. Total US production of the class totalled 'more than 33,000' locomotives of which '12,000 export versions' went to the rest of the world. The 2-6-0, often considered the logical forerunner to the 2-8-0, was first created in the early 1860s.

The first locomotive of this wheel arrangement was likely built by the Pennsylvania Railroad (PRR), but like the first 2-6-0s, this first 2-8-0 had a leading axle that was rigidly attached to the locomotive's frame. To create this 2-8-0, the PRR's master mechanic John P. Laird modified an existing 0-8-0, the Bedford between 1864 and 1865. However, in his book, Swengel claims the first 2-8-0 engine was ordered by Lehigh and Mahanoy Railroad (USA) and that railway named all its engines. The name given to the new locomotive was "Consolidation". Swengel further claimed the Lehigh 2-8-0 evolved from the 0-8-0 locomotives - rather than a 2-6-0.