A legal constant, also called a legal maxim or a maxim of law, is a principle or rule that is widely recognized and accepted as a fundamental and essential part of the legal system. Legal constants are often brief and concise statements that express a fundamental principle of law or a general rule that is applicable across different areas of law. These constants are often used to guide legal reasoning, interpretation, and decision-making.

Here are a few examples of legal constants:

- Ignorantia legis non excusat (ignorance of the law is no excuse): This maxim reflects the principle that individuals are presumed to know the laws of the society in which they live.

- Nullum crimen sine lege (no crime without law): This maxim emphasizes that an act cannot be considered a crime unless it is specifically prohibited by law.

- Pacta sunt servanda (agreements must be kept): This principle underscores the importance of honoring and fulfilling contractual agreements.

- Audi alteram partem (hear the other side): This maxim emphasizes the importance of fair procedures, requiring that all parties in a legal dispute be heard before a decision is made.

- Res ipsa loquitur (the thing speaks for itself): This principle suggests that in certain situations, the circumstances of a case may provide sufficient evidence to prove liability or fault.

- Nemo tenetur seipsum accusare (no one is required to accuse themselves): This principle protects individuals from being compelled to testify against themselves in legal proceedings.

- Ex turpi causa non oritur actio (no legal action arises from a wrongful cause): This maxim disallows parties from seeking legal relief for actions or transactions that are illegal or against public policy.

Legal constants are widely recognized and cited in legal reasoning and decision-making, and they play a significant role in shaping the development and application of the law.