the.com/easter egg

means A hidden feature, message, or joke tucked inside software, a movie, or a game, placed deliberately for attentive users to discover.

from Named by direct analogy to the Easter tradition of hiding decorated eggs for children to hunt — the reward is the same thrill of searching and finding something planted just for you. The term crossed into computing in the late 1970s, reportedly after Atari started hiding programmers' secret messages in game code, treating each discovery like a tucked-away prize waiting in the grass.