Toby is the name of the demon haunting Katie in the Paranormal Activity film franchise.
The first Paranormal Activity film, released in 2007, follows Katie and Micah as they document increasingly disturbing events in their home. Katie says that a demonic presence has affected her since childhood, and the presence is eventually identified by the name Toby.
The franchise uses home-video and surveillance-footage techniques to create the impression of ordinary recordings capturing supernatural activity. Katie's connection to Toby becomes part of the larger mythology involving possession, family history, covens, and attempts to control supernatural forces.
Toby is not the same entity as demons from other modern horror franchises. Bughuul belongs to Sinister, Valak is associated with The Conjuring universe, and Paimon is central to Hereditary. Those names are often mixed up because the films share supernatural-horror themes.