A Kick is when unwanted hydrocarbons, either fluids or gas, enter the well bore, forcing the Drilling Fluids up.
A Kick occurs when pressures within the formation are higher than those in the wellbore. This allows the hydrocarbons to force their way from the formation into the wellbore.
A Kick is normally caused by Drilling Fluids being an inadequate weight to give enough Hydrostatic Pressure within the Wellbore, due to either an oversight by the planners or Mud Engineer. IT can also be caused by movement of the Drill String.
If a Kick is not Killed relatively quickly, it can rapidly escalate to a Blow Out.