Step 1: Writing the standard form.
A rectangular hyperbola has the equation
\[
\frac{x^2}{a^2}-\frac{y^2}{a^2}=1
\] Step 2: Finding eccentricity.
For a hyperbola,
\[
e=\sqrt{1+\frac{b^2}{a^2}}
\]
Here $a=b$. Step 3: Substituting values.
\[
e=\sqrt{1+1}=\sqrt{2}
\] Step 4: Conclusion.
The eccentricity of a rectangular hyperbola is $\sqrt{2}$.