Video Games Encyclopedia

Hack and slash, also known as hack and slay (H&S or HnS) or slash ’em up, refers to a type of gameplay that emphasizes combat with melee-based weapons (such as swords or blades). They may also feature projectile-based weapons as well (such as guns) as secondary weapons. It is a sub-genre of beat ’em up games, which focuses on melee combat, usually with swords.
The term “hack and slash” was originally used to describe a play style in tabletop role-playing games, carrying over from there to MUDs, massively multiplayer online role-playing games, and role-playing video games. In arcade and console style action video games, the term has an entirely different usage, specifically referring to action games with a focus on real-time combat with hand-to-hand weapons as opposed to guns or fists. The two types of hack-and-slash games are largely unrelated, though action role-playing games may combine elements of both.

Reference: Wikipedia