Live Steam refers to the hobby of building and operating model railroad equipment, particularly, but not limited to, steam powered locomotives. Live Steam encompasses scales from 1/2 inch to a foot up to 3 inches to a foot (i.e. models large enough to carry one or more adults). However, smaller scales are also included when using steam powered locomotives.
The term Live Steam was coined in the 1930's, and as such, only included steam operated locomotives. That began to change in the 1970's and 1980's, due to the fact that diesel era models became more popular in the hobby.