Mitre Joint
From DT Online
A Mitre Joint is created by cutting the joining pieces at 45 deg. They are essentially a Butt Joint and therefore quite weak but can be strengthened by reinforcement with Keys (i.e. thin pieces of veneer or similar set into sawn grooves). Keys may be made from the same timber or a contrasting timber (or another material) can create a decorative effect.
Mitre Joints are most commonly seen in picture frames but are useful in any situation where a Rebate, Groove or Moulding has to be returned around a corner.
Note: Several wood joints may include a mitre on one of the faces, especially if there is a need to return mouldings of some kind (e.g. Bridle and Dovetail.