1. [n]  a boat for carrying mail
2. [n]  a small package or bundle
3. [n]  (computer science) a message or message fragment
4. [n]  a collection of things wrapped or boxed together

1. \Pack"et\, n. [F. paquet, dim. fr. LL. paccus, from the
   same source as E. pack. See {Pack}.]
   1. A small pack or package; a little bundle or parcel; as, a
      packet of letters. --Shak.
   2. Originally, a vessel employed by government to convey
      dispatches or mails; hence, a vessel employed in conveying
      dispatches, mails, passengers, and goods, and having fixed
      days of sailing; a mail boat.
   {Packet boat}, {ship}, or {vessel}. See {Packet}, n., 2.
   {Packet day}, the day for mailing letters to go by packet; or
      the sailing day.
   {Packet note} or {post}. See under {Paper}.
   

2. \Pack"et\, v. t. [imp. & p. p. {Packeted}; p. pr. & vb.
   n. {Packeting}.]
   1. To make up into a packet or bundle.
   2. To send in a packet or dispatch vessel.
            Her husband Was packeted to France.   --Ford.
   

3. \Pack"et\, v. i.
   To ply with a packet or dispatch boat.
   

The unit of data sent across a network. "Packet" is a generic term used to describe a unit of data at any layer of the OSI protocol stack, but it is most correctly used to describe application layer data units ("application protocol data unit", APDU).

See also datagram, frame.