HP 33s - Manual (Page 81)

Real–Number Functions 4–17
Names of Functions
You might have noticed that the name of a function appears in the display when
you press and hold the key to execute it. (The name remains displayed for as long
as you hold the key down.) For instance, while pressing
O , the display shows
. "SIN" is the name of the function as it will appear in program lines (and
usually in equations also).