In group theory
Simple group: A nontrivial group that has only two normal subgroups: the whole group and the trivial subgroup.
Term variations: Groupprops:Category:Variations of simplicity
Primary subject wiki entry: Groupprops:Simple group
In noncommutative ring theory
Simple ring: A nonzero unital ring in which the only two-sided ideals are the whole ring and the zero ideal.
Primary subject wiki entry: Noncommutative:Simple ring
Simple space: A path-connected space with Abelian fundamental group whose induced action on all higher homotopy groups is trivial.
Primary subject wiki entry: Topospaces:Simple space
In measure theory
Simple function: A real-valued or complex-valued function on a measure space that is expressible as a finite linear combination of indicator functions of measurable subsets.
Primary subject wiki entry: Measure:Simple function
Also located at: Wikipedia:Simple function