center-line is an interactive compiled Lisp function in `text-mode.el'.

It is bound to M-o M-s.

(center-line &optional NLINES)

Center the line point is on, within the width specified by `fill-column'.
This means adjusting the indentation so that it equals
the distance between the end of the text and `fill-column'.
The argument NLINES says how many lines to center.