In mathematics, the rank, Prüfer rank, or torsion-free rank of an abelian group A is the cardinality of a maximal linearly independent subset. The rank of A determines the size of the largest free abelian group contained in A. If A is torsion-free then it embeds into a vector space over the rational numbers of dimension rank A. For finitely generated abelian groups, rank is a strong invariant and every such group is determined up to isomorphism by its rank and torsion subgroup. Torsion-free abelian groups of rank 1 have been completely classified. However, the theory of abelian groups of higher rank is more involved.

The term rank has a different meaning in the context of elementary abelian groups.

A subset {a_α} of an abelian group A is linearly independent (over Z) if the only linear combination of these elements that is equal to zero is trivial: if

where all but finitely many coefficients n_α are zero (so that the sum is, in effect, finite), then all coefficients are zero. Any two maximal linearly independent sets in A have the same cardinality, which is called the rank of A.

The rank of an abelian group is analogous to the dimension of a vector space. The main difference with the case of vector space is a presence of torsion. An element of an abelian group A is classified as torsion if its order is finite. The set of all torsion elements is a subgroup, called the torsion subgroup and denoted T(A). A group is called torsion-free if it has no non-trivial torsion elements. The factor-group A/T(A) is the unique maximal torsion-free quotient of A and its rank coincides with the rank of A.

The notion of rank with analogous properties can be defined for modules over any integral domain, the case of abelian groups corresponding to modules over Z. For this, see finitely generated module#Generic rank.

Abelian groups of rank greater than 1 are sources of interesting examples. For instance, for every cardinal d there exist torsion-free abelian groups of rank d that are indecomposable, i.e. cannot be expressed as a direct sum of a pair of their proper subgroups. These examples demonstrate that torsion-free abelian group of rank greater than 1 cannot be simply built by direct sums from torsion-free abelian groups of rank 1, whose theory is well understood. Moreover, for every integer ${\displaystyle n\geq 3}$, there is a torsion-free abelian group of rank ${\displaystyle 2n-2}$ that is simultaneously a sum of two indecomposable groups, and a sum of n indecomposable groups.^{[citation needed]} Hence even the number of indecomposable summands of a group of an even rank greater or equal than 4 is not well-defined.

Another result about non-uniqueness of direct sum decompositions is due to A.L.S. Corner: given integers ${\displaystyle n\geq k\geq 1}$, there exists a torsion-free abelian group A of rank n such that for any partition ${\displaystyle n=r_{1}+\cdots +r_{k}}$ into k natural summands, the group A is the direct sum of k indecomposable subgroups of ranks ${\displaystyle r_{1},r_{2},\ldots ,r_{k}}$.^{[citation needed]} Thus the sequence of ranks of indecomposable summands in a certain direct sum decomposition of a torsion-free abelian group of finite rank is very far from being an invariant of A.