A term used in two broad senses.
1. In 1896, US publisher Frank A Munsey (1854-1925) decided that his magazine The Argosy would henceforth print only fiction, for a wide audience. He decided to use the newly available cheap pulp paper, derived from wood pulp in a process invented by Friedrick Gottlob Keller (1816-1895) in 1844 but developed fully only towards the end of the century. The Argosy became the first pulp magazine, or "pulp". Between 1900 and WWII most genre-fiction magazines were pulps.
Pulp paper is thick, porous and friable, and smells; pulps tend to be in large format (usually circa 10in × 7in [25cm × 18cm]), with uncut edges, and they turn yellow within months. Their physical nature precluded their easy acceptance in the tidy US middle-class households in the years preceding 1939.
2. By association, pulp fiction is normally thought of as being lowbrow, action-oriented, pacy, brutal, sexist and ephemeral. There is some truth in this; but not much, and the use of the term to describe a particular type of fiction, regardless of the format of its original publication, has become less and less efficacious.
Pulp magazines are discussed at length in the article Magazines. [JC]