What would the term be for sewing a line of stiching along the edge of a pleat to hold the pleat even through washing? Is there a attchment to help me do this? I have a Singer 201.
If I understand your question correctly, you are in the constructing stage of a garment. In this case, the term would be "basting” your pleats in place.
How ever, your basting stitch does not take place of a regular stitch. It is only to hold your pleats in place until you are ready to place your waistband on etc. You would not need a special foot for this, just a straight stitch foot.
If you just want to sew a straight stitch at the bottom of the pleat to keep the pleats in place for laundering, by all means take the time to do that. You will need your stitch length to be at least 3.0 mm or more. Test your machine for stitch lengths. Usually it is easier to take a pleated garment and have the dry cleaners launder the garment but, which ever works best for you.
I also found an article from Threads Magazine you may find useful. Threads Magazine.
I hope this is helpful to you. Good luck and please don't hesitate to contact me with any other questions you may have.
P.S. Your sewing machine is wonderful. I learned to sew on a machine just like it.