Packet is a standard word used to describe energy, and other things that come in discrete units in science and technology. The internet sends all it's information via discrete packets of data. A big message is broken down into small pieces which are sent separately. If you have a bad internet connection you might be experiencing packet loss - packets that are sent but don't arrive.
A classic physics example is a photon. It's a packet of energy that has a distinct, measurable value. Photons emitted from an atom can't have just any value. It has to be a specific value based on the structure of the atom and the number and position of the electrons.
In reference to electromagnetic energy, packet means a photon. Telecommunications and computer technology have other meanings for the term, as does everyday language, but for straight-up physics it's a photon.