Definition: As Obi-Wan Kenobi said in A New Hope, "The Force can have a strong influence on the weak-minded." Jedi use mind tricks -- a.k.a. "affect mind" or "alter mind" -- to influence others using the Force. While the Jedi mind trick most familiar from the movies is the power of Force suggestion, other mind tricks include creating illusions or controlling someone's mind.
The phrase itself comes from Return of the Jedi, in which Jabba the Hutt upbraids his major domo Bib Fortuna for his susceptibility to the "old Jedi mind trick" being performed on him by Luke Skywalker. Though this is a general description rather than a Jedi technical term, it has become the phrase normally used to describe Force influence on the minds of others.
In-universe: By using a Jedi mind trick, a Force user can block a creature's perception of its surroundings and plant a new suggestion. The effects of a Jedi mind trick range from simple persuasion -- for example, convincing a guard he hasn't seen anything suspicious -- to illusions affecting a group -- for example, an army perceiving a larger enemy force than really exists.
A successful Jedi mind trick requires good powers of perception. A Force user must be able to reach into a subject's mind and learn the best way to affect him. For example, creating an illusion of a larger army will not help matters if the enemy will be motivated to fight harder against a larger force.
The Jedi prefer non-violent solutions when possible, and see the Jedi mind trick as a way to get out of situations without fighting. Misusing a mind trick, however, can lead to the dark side. Some Sith went beyond merely planting suggestions, attempting instead to take complete control of the subject's mind.
Yarael Poof, a master of Jedi mind tricks, warned Jedi to be mindful of less obvious problems stemming from the use of Jedi mind tricks. For example, he cautioned Jedi to consider that convincing a guard to let you pass could cost him his job, or that convincing him to chase an illusion could lead to injury.
Some species, including Hutts and Toydarians, are naturally resistant or immune to Jedi mind tricks as a result of their brain structure. Other creatures could learn to resist Jedi mind tricks with training.
Read more: Jedi vs. Sith: The Essential Guide to the Force by Ryder Windham (2007)