Synonyms of the word

hard-and-fast

• adj. (idiomatic, of rules, etc) strictly maintained.

invariable

• adj. Not variable; unalterable; uniform; always having the same value.
• adj. (mathematics) Constant.
• adj. (by extension, grammar, of a word, or a grammatical class) That cannot undergo inflection, conjugation…
• n. Something that does not vary; a constant.

strict

• adj. Strained; drawn close; tight.
• adj. Tense; not relaxed.
• adj. Exact; accurate; precise; rigorously nice.
• adj. Governed or governing by exact rules; observing exact rules; severe; rigorous.
• adj. Rigidly interpreted; exactly limited; confined; restricted.
• adj. (botany) Upright, or straight and narrow; — said of the shape of the plants or their flower clusters.
• adj. Severe in discipline.