variant
This term carries a technical and clinical weight, often used in scientific, linguistic, or industrial contexts to describe a deviation from a baseline. It suggests a relationship of kinship; a variant is not a completely different entity, but a modification of a known original. It is far more precise than the word "different," which is too broad for these specific contexts.
In modern usage, the word has become heavily associated with epidemiology and genetics. When used in casual conversation, it often implies a slight mutation or a specialized version of a product, such as a software variant or a regional dialect variant, maintaining a sense of structured variation rather than random chaos.
Countable when referring to a specific version of a thing, such as a new variant of a virus or a variant of a spell.