Flexible narrow strips of featherboning or stiff nylon strip, used to support edges or seams of close-fitting garments to prevent slipping (e.g. in bodices, strapless dresses)
To expand the definiton of boning, here is a direct quote from my book The Basics Of Corset Building – A Handbook for beginners by Linda Sparks:
Bones are also known as stays or steels. Originally there was whalebone, reeds, or steel that rusted. Today there are three main types of bones and each type can be broken into sizes and subtypes. These three moderm Alternatives are:
-plastic – solid and woven
-spring steel/white bones
For more info get this book or message me.