A steady flow is characterized by constant values of all flow variables at any location in space. If $\phi$ is any flow quantity like velocity, pressure, etc., then in steady flow

$\frac{\partial \phi}{\partial t}(x,y,z,t) \equiv 0$

Hence all flow quantities depend only on the spatial location

$\phi(x,y,z,t) = \phi(x,y,z)$

where as a flow in which flow/fluid properties keep on changing with time, is termed as unsteady flow.