A Lie group is a group (in the sense of abstract algebra) that is also a differentiable manifold, such that the group operations (addition and inversion) are smooth, and so we can study them with differential calculus. They are a special type of topological group.
Consider using with the (group-theory) tag.
Lie groups are groups that are also differentiable manifolds that represent the best developed theory of continuous symmetry of mathematical objects.
Examples of lie groups are:
1) The Euclidean space $\mathbb{R}^n$ under addition is a lie group.
2) The special orthogonal group of real orthogonal matrices with determinant $1$ (note that $n=3$ is the rotation group in $\mathbb{R}^3$).
3) The spin group, which is the double cover of the special orthogonal group such that $\exists$ a sequence of lie groups:
\begin{equation*} 1\to Z_2\to~\text{Spin}(n)\to SO(n)\to 1. \end{equation*}
Note that it has dimension $\frac{n(n-1)}{2}.$