A transporter is a teleportation machine used in the Star Trek universe. Transporters convert a person or object into an energy pattern (a process called dematerialization), then "beam" it to a target, where it is reconverted into matter (rematerialization). The term transporter accident is a catch-all term for when a person or object does not rematerialize correctly.
According to dialogue in the Star Trek: Enterprise episode "Daedalus", the transporter was invented in the early 22nd century by Dr. Emory Erickson, who also became the first human to be successfully transported. Although the Enterprise (NX-01) has a transporter, the crew does not routinely use it (Captain Jonathan Archer once said that he wouldn't even put his dog through it), generally preferring shuttlepods or other means of transportation before falling back on the transporter if no other means of transportation were possible or feasible. The crew aboard the 23rd century USS Enterprise frequently use the transporter. By the 24th century, transporter travel was very reliable and "the safest way to travel", according to dialogue in the Star Trek: The Next Generation episode "Realm of Fear".
According to the Star Trek: Deep Space Nine episode "Homefront", Starfleet Academy cadets receive transporter rations.
Despite its frequent use, characters such as Leonard McCoy and Katherine Pulaski are reluctant to use the transporter, as the characters express in the Next Generation episodes "Encounter at Farpoint" and "Unnatural Selection", respectively. Additionally, Reginald Barclay expresses his outright fear of transporting in "Realm of Fear".