Someone whose job is welding — joining pieces of metal by heating them until they melt together at the join. Welders work in factories, on ships, on building sites, and on oil rigs. It is a skilled trade: a good welder trains for years, needs a certificate in many countries, and can earn very well, especially for dangerous work like underwater welding. Note one thing that trips learners up: 'welder' also names the machine, not just the person. 'I bought a welder last week' means you bought the welding equipment. Which one you mean is almost always clear from the sentence — machines get bought and plugged in, people get hired and go home. English does this with a lot of -er words (a cooker, a printer, a driver in a golf bag), so it is worth noticing the pattern rather than learning each case separately.