Instantaneous velocity is the gradient of a displacement-time graph at a point

Motion Along a Straight Line — AQA A-Level Physics

Instantaneous velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹. is the velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹. at a single moment in time.
On a displacementThe distance moved in a particular direction from a starting point. A vector quantity. Measured in metres (m).-time graph, draw a tangent at the required time and calculate its gradient.
Average $velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹. = total displacementThe distance moved in a particular direction from a starting point. A vector quantity. Measured in metres (m). divided$ by total time.
For a straight line on a displacementThe distance moved in a particular direction from a starting point. A vector quantity. Measured in metres (m).-time graph, instantaneous and average velocity are the same (constant velocity).