invariable

Use the adjective invariable to describe something that's unlikely to change, such as your invariable custom of eating deep-dish pizza every Saturday.

You can hear the word vary in invariable. Something that varies changes or is made up of different kinds of things. When you add the prefix in-, meaning "not," and the adjective suffix -able, you get an adjective to describe something that's not able to vary or change. Synonyms for invariable include constant, fixed, and immutable. Someone who's invariable in her habits may seem predictable and boring on the one hand, and steady and reliable on the other.

Definitions of invariable
  1. adjective
    not liable to or capable of change
    “an invariable temperature”
    “an invariable rule”
    “his invariable courtesy”
    Synonyms:
    consistent
    (sometimes followed by `with') in agreement or consistent or reliable
    constant
    steadfast in purpose or devotion or affection
    changeless, constant, invariant, unvarying
    unvarying in nature
    hard-and-fast, strict
    (of rules) stringently enforced
    invariant
    unaffected by a designated operation or transformation
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    Antonyms:
    variable
    liable to or capable of change
    inconsistent
    displaying a lack of consistency
    inconstant
    likely to change frequently often without apparent or cogent reason; variable
    changeable, uncertain, unsettled
    subject to change
    covariant
    changing so that interrelations with another variable quantity or set of quantities remain unchanged
    multivariate
    pertaining to any procedure involving two or more variables
    protean
    taking on different forms
    shifting
    continuously varying
    variant
    exhibiting variation and change
    versatile
    changeable or inconstant
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  2. noun
    a quantity that does not vary
    synonyms: constant, constant quantity
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    types:
    parameter, parametric quantity
    a constant in the equation of a curve that can be varied to yield a family of similar curves
    degree of freedom
    one of the minimum number of parameters needed to describe the state of a physical system
    type of:
    quantity
    the concept that something has a magnitude and can be represented in mathematical expressions by a constant or a variable
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