Kinktionary
More like guidelines and a lot less like definitions.
Masochism
Masochism, also known as sexual masochism, is on of the core principles of BDSM as an opposite to Sadism. The M in BDSM stands for masochism.
It's an umbrella term for all forms of play that result in a person receiving pain and perceiving that as pleasurable or arousing. People who enjoy receiving pain are often known as Masochists or algophiles.
Origins of the Term
In the 1800s Leopold von Sacher-Masoch wrote novels containing his masochistic fantasies and it is from his surname that we get the term masochism.
Krafft Ebing helped to popularize the term when he wrote about Masochism in his 1886 work on sexual psychopathy called Psychopathia Sexualis.