X-gate (or Pauli-X gate / Quantum NOT gate) is a 1-qubit quantum gate that serves as the quantum equivalent of logical NOT. It's represented by the following 2×2 matrix.

$$ X = \begin{pmatrix}0 & 1\\ 1 & 0\end{pmatrix}$$

Applying the X gate to the qubit $\lvert 0\rangle$ yields $\lvert 1\rangle$, and applying X to the qubit $\lvert 1\rangle$ yields $\lvert 0\rangle$.

$$X = \begin{pmatrix}0 & 1\\1 & 0\end{pmatrix}$$