In thermodynamics, first law is shorthand for the first law of thermodynamics along with other synonyms, including: the first main principle as well as, loosely speaking, the conservation of energy; although, technically speaking, this is more of a step in the derivation of the first law.
See also
● Zeroth law
● Second law
● Third law
● Forth law
● Joule’s first law
● Newton’s first law
References
1. Koltz, Irving and Rosenberg, Robert M. (2008). Chemical Thermodynamics (pg. 81). Wiley.