Any locally compact Hausdorff topological group has a unique (up to scalars) nonzero left invariant measure which is finite on compact sets. If the group is Abelian or compact, then this measure is also right invariant and is known as the Haar measure.
More formally, let 
be a locally compact group. Then a left invariant Haar measure on 
is a Borel measure 
satisfying the following conditions:
1. 
for every 
and every measurable 
.
2. 
for every nonempty open set
.
3. 
for every compact set 
.
For example, the Lebesgue measure is an invariant Haar measure on real numbers.
In addition, if 
is an (algebraic) group, then 
with the discrete topology is a locally compact group. A left
invariant Haar measure on 
is the counting measure on 
.
