The phrase "fixed entity" is correct and usable in written English.
It can be used in contexts where you want to describe something that is stable, unchanging, or defined.
Example: "In the realm of software development, a fixed entity refers to a component that does not change throughout the lifecycle of the application."
Alternatives: "static entity" or "immutable object."
Family is less a fixed entity than a kind of weather system that keeps coming back.
Surrendering control of the work's structure to individual online visitors challenges the notion of a classical composition as a fixed entity.
These distinctions assist the critic in judging each mode according to its proper ends instead of regarding beauty as a fixed entity.
His subject is the English avant-garde composer Cornelius Cardew (1936-81), who in 1968 helped found the Scratch Orchestra, a music ensemble that was less a fixed entity than an ideal, a kind of communal jam session in a movable-feast format.
The flaw in the melting pot definition of America is its tacit assumption that acculturation only takes places in one direction, and that the country that newcomers assimilate into is a static, fixed entity.
Health is not a fixed entity.
Haslam et al. (2012) present a case for moving away from a conceptualization of personality traits as deterministic (i.e., fixed entities that interact with fixed properties of the environment to produce fixed health behaviour) towards a dynamic interactionism whereby the individual is changed through the nature of social interaction.
Species are not fixed entities created at one time.
Receptive fields are changing and not fixed entities.
The approach is easy to use, effective and can readily accommodate varying degrees of preference regarding adjacencies and fixed entities.
An important message for conservation is that neither the environment nor species are fixed entities, therefore a wise strategy for conserving endangered species should keep them capable of further change.
