an imaginary body of air a few meters in diameter which possesses nearly uniform properties within it
An imaginary body of air to which may be assigned any or all of the basic dynamic and thermodynamic properties of atmospheric air.
An air parcel is a meteorologically defined chunk of air in the atmosphere. How large or small this chunk is depends on scale you are looking at. Meteorologists study the movements and changes in air parcels over time as one tool in weather forecasting.