Source: @baez1994gauge page 197.
Recall that a group representation of on a vector space is a homomorphism . Similarly, a representation of a Lie algebra on is defined as a Lie algebra homomorphism , where denotes the Lie algebra of linear operators on under the commutator bracket. It is clear that differentiating a representation yields a representation of the corresponding Lie algebra. Conversely, a deeper result asserts that if is a representation of a simply connected Lie group , it can be “exponentiated” to obtain a representation such that .