The general absence of agitation or disturbance.
The general absence of agitation or disturbance — the broad, neutral baseline of the family.
The broad, general-purpose, least-loaded term: the absence of agitation, of a person, a sea, the weather, or a situation, and either momentary or settled. It does not by itself imply serenity's depth, equanimity's under-pressure balance, composure's willed self-control, or tranquility's restful stillness — each of those adds a specific shade to the basic notion calmness names. The plain default; reach for the others when you need the nuance.
calm + -ness. Calm (late 14c., of the sea “windless, without agitation”) is probably from Late Latin cauma “heat of the midday sun” (a time of rest), from Greek kauma “heat.”
Calm originally described the sea/weather; the figurative application to mind and temper is from the mid-16th century — the same weather-to-mind shift as serene. No reliable recent-generation shift.