A cosmetic weld is a resistance weld on a surface where a typical weld indentation would not be permitted. The middle of the hood of your car is a good example. Welds on medical instruments would be another case. To prevent these indentations from showing most of the heat input must be made from the other side of the part.
Generally “C” flat faced electrodes are used on the cosmetic side and care is taken to not over heat the part. Usually the weld nuggets are run in the lower range of the quality specification to reduce the heat affected area.