In railway terminology, a single is a steam locomotive with a single pair of driving wheels. Some sources use 'Single' only for the 2-2-2 type, but more commonly singles could have any number of leading or trailing wheels.
See also Caledonian Railway Single Crampton locomotive GWR 3031 Class GNR Stirling 4-2-2 Midland Railway 115 Class
References illustrated description of some famous singles