The Collaborative International Dictionary of English v.0.48:

Easter \Eas"ter\ ([=e]s"t[~e]r), n. [AS. e['a]ster, e['a]stran,
   paschal feast, Easter; akin to G. ostern; fr. AS. E['a]stre,
   a goddess of light or spring, in honor of whom a festival was
   celebrated in April; whence this month was called in AS.
   E['a]sterm[=o]na[eth]. From the root of E. east. See East.]
   1. An annual church festival commemorating Christ's
      resurrection, and occurring on Sunday, the second day
      after Good Friday. It corresponds to the pascha or
      passover of the Jews, and most nations still give it this
      name under the various forms of pascha, pasque,
      p[^a]que, or pask.
      [1913 Webster]

   2. The day on which the festival is observed; Easter day.
      [1913 Webster]

   Note: Easter is used either adjectively or as the first
         element of a compound; as, Easter day or Easter-day,
         Easter Sunday, Easter week, Easter gifts, Easter eggs.
         [1913 Webster]

               Sundays by thee more glorious break,
               An Easter day in every week.       --Keble.
         [1913 Webster]

   Note: Easter day, on which the rest of the movable feasts
         depend, is always the first Sunday after the fourteenth
         day of the calendar moon which (fourteenth day) falls
         on, or next after, the 21st of March, according to the
         rules laid down for the construction of the calendar;
         so that if the fourteenth day happen on a Sunday,
         Easter day is the Sunday after. --Eng. Cyc.
         [1913 Webster]

   Easter dues (Ch. of Eng.), money due to the clergy at
      Easter, formerly paid in communication of the tithe for
      personal labor and subject to exaction. For Easter dues,
      Easter offerings, voluntary gifts, have been substituted.


   Easter egg.
      (a) A painted or colored egg used as a present at Easter.
      (b) An imitation of an egg, in sugar or some fine
          material, sometimes made to serve as a box for jewelry
          or the like, used as an Easter present.
          [1913 Webster]

WordNet (r) 3.0 (2006):

Easter egg
    n 1: an egg-shaped candy used to celebrate Easter
    2: a colored hard-boiled egg used to celebrate Easter

The Jargon File (version 4.4.7, 29 Dec 2003):

Easter egg
 n.

    [from the custom of the Easter Egg hunt observed in the U.S. and many parts
    of Europe]

    1. A message hidden in the object code of a program as a joke, intended to
    be found by persons disassembling or browsing the code.

    2. A message, graphic, or sound effect emitted by a program (or, on a PC,
    the BIOS ROM) in response to some undocumented set of commands or
    keystrokes, intended as a joke or to display program credits. One
    well-known early Easter egg found in a couple of OSes caused them to
    respond to the command make love with not war?. Many personal computers
    have much more elaborate eggs hidden in ROM, including lists of the
    developers' names, political exhortations, snatches of music, and (in one
    case) graphics images of the entire development team.


The Free On-line Dictionary of Computing (30 December 2018):

easter egg

    (From the custom of the Easter Egg hunt observed in
   the US and many parts of Europe)

   1. A message hidden in the object code of a program as a
   joke, intended to be found by persons disassembling or
   browsing the code.

   2. A message, graphic, sound effect, or other behaviour
   emitted by a program (or, on an IBM PC, the BIOS ROM) in
   response to some undocumented set of commands or keystrokes,
   intended as a joke or to display program credits.

   One well-known early Easter egg found in a couple of
   operating systems caused them to respond to the command
   "make love" with "not war?".  Many personal computers, and
   even satellite control computers, have much more elaborate
   eggs hidden in ROM, including lists of the developers' names
   (e.g. Microsoft Windows 3.1x), political exhortations and
   snatches of music.  The Tandy Color Computer 3 (CoCo) had
   images of the entire development team.  Microsoft Excel 97
   includes a flight simulator!

   (http://eeggs.com/).

   [Jargon File]

   (2003-06-23)