Linked from
The 29 pages that link to Fundamental group, each with the reason it gives.
TopologyRelated: It detects certain holes by recording how loops can or cannot contract.
Topological spaceRelated: It extracts algebraic information about loops from a space's topology.
SubgroupRelated: Its subgroups encode covering spaces of a connected, suitably well-behaved space.
Simply connected spaceRelated: A space is simply connected exactly when this group is trivial.
FunctorRelated: The fundamental group construction maps continuous maps to group homomorphisms.
TorusRelated: Its fundamental group is the free abelian group on two generators.
Branch pointRelated: Loops around deleted branch points determine the continuation transformations.
Equivalence classRelated: Its elements are equivalence classes of loops under homotopy.
SU(2)Related: SU(2) is simply connected, whereas SO(3) has a nontrivial fundamental group.