Boning
Rigid or semi-rigid strips inserted into fabric channels to give a bodice or corset structure, ranging from flexible spiral steel to rigid plastic.
"Boning" is a holdover name from when the actual material was baleen — the flexible keratin plates from a whale's mouth, not bone at all, but the term stuck long after baleen fell out of use. What you buy today is almost always one of a few modern substitutes, and they behave differently enough that the choice matters.
Spiral steel boning is flexible in every direction, curves with the body, and is what I reach for on most fitted bodices — it moves with you rather than fighting you. Flat steel boning bends in one direction only and stays rigid in the other, which makes it the right choice for the front busk or anywhere you want zero curve. Plastic (often called featherweight or rigilene) boning is the cheapest and easiest to cut with ordinary scissors, but it softens with heat and body warmth over a long day of wear, so I only use it on lighter-support garments, not on anything doing real structural work like a corset.
Whichever type, the cut ends need finishing before they ever go in a channel — steel ends get dipped in a boning tip solution or capped with metal end caps; plastic can just be filed smooth. Skip this and the raw end will eventually push through the fabric.