An LTI system is "initially relaxed" or "at rest" if all its initial conditions are zero before an input is applied. This means there is no stored energy in the system.
A consequence of being initially relaxed is that if the input to the system is zero for all time (\(x(t)=0\) or \(x[n]=0\)), then the output will also be zero for all time (\(y(t)=0\) or \(y[n]=0\)). This is known as the zero-input zero-output (ZIZO) property for a system at rest.
If a zero input produces a non-zero output, it indicates the presence of non-zero initial conditions.
\[ \boxed{\text{Zero input produces zero output}} \]