Alternate Reality(split 2233)
A particle lock was a component of the transporter used by Starfleet in the mid-23rd century. It required the object or person to be stationary during transportation.
During the battle of Vulcan, Hikaru Sulu and James T. Kirk fell from the Narada's drilling rig, making it difficult to lock onto them. Pavel Chekov was able to engage the lock in time before they hit the ground. (Star Trek)
The term comes from a graphic on the transporter operations console. It is unknown if the Prime universe's transporters also involve this component.