From AMS Glossary
An imaginary volume of air to which may be assigned any or all of the basic dynamic and thermodynamic properties of atmospheric air.
A parcel is large enough to contain a very great number of molecules, but small enough so that the properties assigned to it are approximately uniform within it and so that its motions with respect to the surrounding atmosphere do not induce marked compensatory movements. It cannot be given precise numerical definition, but a cubic foot of air might fit well into most contexts where air parcels are discussed, particularly those related to static stability. Any fluid parcel may be defined similarly.