Cord strength is a term used in the application of O-rings as a sealing element. O-rings have a circular cross-section, the diameter of which is typically referred to as the cord thickness.
The round cross-section distinguishes O-rings from other common sealing elements with a rectangular cross-section (flat gaskets). Together with the inside diameter, the cord thickness serves as a complete geometric description of an O-ring.
The format “inner diameter x cord thickness” is a common ordering specification for standard O-rings.