fastness

Fastness is a noun with several related meanings derived from the adjective fast, meaning firm, secure, or fixed. In general use, it denotes the quality of being steadfast or durable, and in some contexts it refers to a fortress or stronghold.

The most common modern usage is in textiles, where color fastness refers to the resistance of dyed or printed fabrics to fading or color transfer under conditions such as washing, light exposure, rubbing, perspiration, or chemical action. Evaluation follows standardized tests and rating scales (for example, colorfast to washing, to light, to rubbing). Factors affecting color fastness include the dye chemistry, fiber type, finishing agents, and laundering conditions. Manufacturers report fastness values to guide consumers and designers in choosing fabrics for specific uses.

Historically, fastness also meant a fortress or stronghold—the place in a castle or town designed to resist assault. In this sense, it appears in historical and literary texts and place-names, where a fastness denotes a defensible position. In other specialized uses, the term can appear in philosophy or law to describe the state of being secure or fixed, though this is less common. The concept is closely related to durability, resilience, and security, depending on context.