Letters standing for fixed quantities of nature. Grouped because of an unusual property: since the 2019 SI redefinition several of these have values that are exact by definition rather than measured, and the units are now defined from the constants instead of the other way round. The gravitational constant is the exception and the interesting one — still measured, still the least precisely known of the major constants, with independent experiments disagreeing beyond their stated uncertainties.
An italic lower-case c, denoting the speed of light in vacuum.
An italic lower-case h, denoting the quantum of action.
An italic k, often subscripted B, relating temperature to energy.
A capital N with subscript A, the number of entities in a mole.
An italic lower-case e, the magnitude of the charge on a proton.
An italic capital G, the constant in Newton's law of gravitation.
A Greek lambda, denoting the distance between successive wave peaks.
A t with subscript one-half, the time for half a quantity to decay.
An element symbol with mass number above and atomic number below, to its left.