A Lie Algebra is a Vector Space with an operation, a Lie Bracket, that satisfies the Jacobi Identity.
Jacobi Identity
A set A with two binary operations + and × , with an additive identity 0 will satisfy the Jacobi Identity if for all x, y, z in A, the following is true:
x×(y×z) + y×(z×x) + z×(x×y)=0
Lie Bracket
One example of a Lie Bracket operation is the cross product:
[x,y] = x × y