The apex of 19th century research into the behavior of gasses. Also incorporates the molecular nature of gasses.
PV=nRT, where P is the pressure, V is the volume, n is the number of molecules of gas, T is the temperature (measured from absolute zero), and R is a constant which cancels out the units from the other terms. What R actually is is determined by the units you use for the other terms, so if you wanted, you could do the whole equation in millimeters of Hg, pounds per square inch, and Farenheit. But the math is a pain in the ass unless you use the usual.
This law combines the three older gas laws: Boyle's Law, Charles' Law, and Gay-Lussac's Law, and incorporates a term reflecting the number of molecules of gas.
I remembered the Ideal Gas Laws via the mnemonic "BOTCAPGOV", where Boyle's Law holds "T"emperature constant, Charles' Law holds "P"ressure constant, and Gay-Lussac's Law holds "V"olume constant. "PV=nRT" is easy to remember as "PerVNeRT" . I still remember all these from high school, many years ago. Use these mnemonics with caution.