Definitions of constant:

• noun:   a quantity that does not vary
• noun:   a number representing a quantity assumed to have a fixed value in a specified mathematical context

  Example: "The velocity of light is a constant"

• adjective:   steadfast in purpose or devotion or affection

  Example: "A man constant in adherence to his ideals"

• adjective:   continually recurring or continuing without interruption

  Example: "Constant repetition of the exercise"

• adjective:   persistent in occurrence and unvarying in nature

  Example: "Maintained a constant temperature"

• adjective:   uninterrupted in time and indefinitely long continuing

  Example: "In constant pain"

• name:  A surname (rare: 1 in 50000 families; popularity rank in the U.S.: #6501)