A warp ellipse was a temporal and space-based hazard that could capture and disrupt the flight of spacecraft and time travel vessels. It could lock craft in a fixed orbit in time and space. Escaping from a warp ellipse could cause temporal anomalies.
Mawdryn's spacecraft entered a warp ellipse 3000 years prior to the 20th century. However, the ship was close enough to Earth to enable its transmat system to function, enabling them to set up a transmat relay system on Earth. (TV: Mawdryn Undead)