The cotree 
of a spanning tree 
in a connected graph 
is the spacing subgraph of 
containing exactly those edges of 
which are not in 
(Harary 1994, p. 39).
Cotree
See also
TwigExplore with Wolfram|Alpha
References
Harary, F. Graph Theory. Reading, MA: Addison-Wesley, 1994.Referenced on Wolfram|Alpha
CotreeCite this as:
Weisstein, Eric W. "Cotree." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Cotree.html