Class L dwarfs are dwarf stars that are cooler than M class stars. They are on the borderline of supporting fusion of hydrogen in their cores, and their temperatures range from 1,300 K to 2,400 K, cool enough to have alkaline metals and metal hydrides in their atmospheres.
The term dwarf star refers to a variety of distinct classes of stars. The term was originally coined in 1906 when the Danish astronomer Ejnar Hertzsprung noticed that the reddest stars - classified as K and M in the Harvard scheme - could be divided into two distinct groups. They are either much brighter than the Sun, or much fainter. To distinguish these groups, he called them "giant" and "dwarf" stars, the dwarf stars being fainter and the giants being brighter than the sun.