3 KudoZ points awarded for this answer. Comment: "A true case of split points here. I think Yves’ answer contributed largely, although according to his source, the « pass-by-pass » suggestion is apparently an inspection technique. Paola’s terms were confirmed by further searches : root pass, single pass, multi/multuiple pass. In the end, after all these leads, I found a welding theory and application glossary (http://www.engineersedge.com/welding.htm : « pass : The weld metal deposited in one general progression along the axis of the weld ») which seems reliable and matches am1971’s answer. I also found sources referring to « successive passes » which I shall use as this is for children. "