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Single qubit Hadamard gate transforms standard basis states (zero and one states) to their superpositions (plus and minus states)

Single qubit Hadamard gate transforms $\mid 0\rangle$ state to $|+\rangle=\frac{\mid 0\rangle + \mid 1\rangle}{\sqrt{2}}$ state and $\mid 1\rangle$ state to $|-\rangle=\frac{\mid 0\rangle - \mid 1\rangle} {\sqrt{2}}$ state and so, in matrix form, can be written as $$H = |+\rangle\langle 0|+|-\rangle\langle 1|= \frac{1}{\sqrt{2}}\begin{pmatrix}1&1 \\\ 1&-1\end{pmatrix}$$.curcuit symbol for Hadamard gate taken from

The $n$-qubit Hadamard gate can then be written as $$H^{\otimes n} = H\otimes H^{\otimes \left(n-1\right)}$$ and is $n$ individual Hadamard gates acting on $n$ different qubits.

For more info, one can visit Hadamard matrix and Hadamard transform.