在数学里，卡西米尔不变量或卡希米尔算子是李代数的泛包络代数中心的一个显著元素。最典型的例子是角动量算符的平方，一个三维旋转群的卡西米尔不变量。

In mathematics, a Casimir element (also known as a Casimir invariant or Casimir operator) is a distinguished element of the center of the universal enveloping algebra of a Lie algebra. A prototypical example is the squared angular momentum operator, which is a Casimir element of the three-dimensional rotation group.