An "invertible" object or function is one that can be reversed or undone. In mathematics, a function is considered invertible if there exists another function that can take the output of the original function and return it to the original input. For example, the function f(x) = 2x is invertible because you can find its inverse, f⁻¹(x) = x/2. In linear algebra, a matrix is invertible if there exists another matrix, called the inverse, that when multiplied together results in the identity matrix. An invertible matrix has a non-zero determinant, which indicates that it has a unique solution for every input.