look up any word, like half chub:
The stripe that is on the underside of the penis where it looks like the skin has been joined together.

If you check your own penis you will see it - and as it has no official medical term, and has appearance the of a seam weld, this is commonly referred to as 'The Weld'.
Look at ' (The) weld' on that cock
by Les of Lesbania November 12, 2010