Home-made joint for a top window
If you're refering to the sort of caulk-like stuff that runs along the edge of the glass, it's called 'window glazing putty', sometimes 'window glazing compound'.

You can find sites online that cover how to re-glaze a window; you may want to remove the glazing from the rest of the window so you can get a clean edge all the way around, and a matching color:


http://www.hereandthere.org/oldhouse/windows-glazing-steps.html (has pictures)
http://www.naturalhandyman.com/iip/infwindows/infgla.html (also covers removing old glazing)