C21

C21 is a designation used across multiple fields to denote items associated with the number 21. It does not refer to a single entity, but rather to a variety of models, codes, or descriptors whose meaning depends on the context.

In military aviation, C-21 refers to the Learjet 35A-based transport used by the United States Air Force for personnel movement and sometimes medical evacuation. The designation highlights the aircraft’s role as a rapid, point-to-point transport within the military fleet.

In chemistry, C21 commonly appears as shorthand for molecules containing 21 carbon atoms. For example, the alkane with 21 carbons has the formula C21H44. The C21 designation can also underpin the carbon skeleton of other 21-carbon compounds encountered in organic chemistry.

In biochemistry, C21 steroids are a class of steroids with 21 carbon atoms in their core structure. Notable examples include progesterone (C21H30O2) and cortisol (C21H30O5), both of which are important hormones with distinct physiological roles.

Because C21 is a generic label, its precise meaning is highly context-dependent. Without domain-specific information, it can refer to a vehicle designation, a molecular descriptor, or a structural classification, among other possibilities.