What is the term used in tennis when the score is 40-40?

When both sides have won the same number of points then: when each side has won one, or two, points, the score is described as "15-all" and "30-all" (or "15-up" and "30-up"), respectively.

However, if each player has won three points, the score is called as "deuce", not "40?all". From that point on in the game, whenever the score is tied, it is described as "deuce", regardless of how many points have been played.