As a noun entertainment
is an activity designed to give pleasure, enjoyment, diversion, amusement, or relaxation to an audience, no matter whether the audience participates passively as in watching opera or a movie, or actively as in games.
As a letter d is
the fourth letter of the.
As a numeral d is
cardinal number five hundred (500).
As a symbol d is
deuterium, when it needs to be distinguished from ordinary hydrogen.