Definition of extensions:
(n) :
The act of extending; a stretching out; enlargement in length, breadth, or time; an increase.
(n) :
The state of being extended.
(n) :
That property of a body by which it occupies a portion of space (or time, e.g. "spatiotemporal extension").
(n) :
(mathematics, group theory, of a group G by a group H) A short exact sequence 1→H→E→G→1, or the group E therein.
(n) :
(mathematics, homological algebra, of an object A by an object B in an abelian category) A short exact sequence 0→B→E→A→0, or the object E therein.
(n) :
A part of a building that has been added onto the original.