A time fissure was a weakness in time and space, a rent in the fabric of the Web of Time itself.
Sarah Jane Smith sends "Oscar" home through a Time Fissure (TV: The Temptation of Sarah Jane Smith)
Structure
Described as a "crack in Time" by Charley Pollard, a time fissure could rupture at one point in time and stretch to another point through the Time Vortex, possibly sending temporal waves outward as a result. One time fissure, which may have been an artificially-created temporal paradox, ruptured in the mid-21st century and ended in the 16th century. Whether that meant that anyone in that 400+ year span could disappear is unknown. (AUDIO: The Time of the Daleks)
A person who stepped into a time fissure would become lost in time. (TV: Image of the Fendahl) It may be, then, that a person could not exit a time fissure like they could a time rift.
Common effects
The Moment creating a time fissure, allowing the War Doctor to enter his future. (TV: The Day of the Doctor)
Time fissues occurred often and created divergent realities (parallel universes or alternate timelines); most were tiny and barely affected the space-time continuum, but large events such as the sinking of the Titanic "created" large fissures. Crossing a fissure via its nexus point allowed someone to cross from one reality into another. (AUDIO: The Wreck of the Titan)
Places that were reputed to be haunted often had a time fissure nearby. (TV: Image of the Fendahl) They were also often responsible for disappearances, as people accidentally wandered through them and got lost in time. (TV: The Temptation of Sarah Jane Smith) Growing up near a time fissure tended to enable one to develop psychic powers such as precognition and "second sight". (TV: Image of the Fendahl)
History
The English village of Fetchborough was situated in or near a time fissure. Consequently and because of her long exposure to the fissure, a local, Martha Tyler had a reputation as a witch. The presence of the fissure made Fendelman's 1970s experiments with "Eustace", the skull of a Fendahl Core, even more dangerous. (TV: Image of the Fendahl)
Sarah Jane Smith, Mike Yates and K9 Mark III visited a house built over a time fissure. (PROSE: Housewarming)
A time fissure opened seven miles from the former site of Foxgrove; connecting London in 2008 to Foxgrove in 1951. Sarah Jane Smith and her team investigated it and Sarah Jane used a device to open and close the fissure. When activated, the fissure manifested as a glowing portal through which people could pass. It is unclear whether this time fissure opened naturally, or if it was deliberately opened by the Trickster as part of the trap to get Sarah Jane to prevent her late parents' deaths. (TV: The Temptation of Sarah Jane Smith)
The Moment opened a Time fissure to show to the War Doctor his future incarnations in an effort to stop him from destroying Gallifrey. (TV: The Day of the Doctor)