(linear algebra) An explicit formula for the solution of a system of linear equations with as many equations as unknowns, valid whenever the system has a unique solution.

(evolutionary biology) The principle of biology that states that populations isolated onto islands may undergo evolution towards dwarfism or gigantism to better conform to available resources

(poker) The rule of multiplying the number of outs by four to discover one's approximate chance to make one's hand on the turn or river. Compare rule of two.

(cellular automata) A simple cellular automaton, unusual in being Turing-complete, where each cell's value changes depending on its current value and those of two neighbouring cells.

(mathematics) The rule that the limit of the ratio of two functions equals the limit of the ratio of their derivatives, usable when the former limit is indeterminate and the latter limit exists.

(mathematics) A formula used to find the derivatives of products of two or more functions: (f·g)'=f'·g+f·g',!

Archaic form of rule the roost.

A scale marked with numbers for finding without calculation the number of square feet in a board.

(engineering) A knuckle joint having shoulders that abut when the connected pieces come into line with each other, and thus permit folding in one direction only.

(printing, historical) A thin slip of brass or steel, against which the type is arranged in a composing stick, or by the aid of which stickfuls or handfuls or type are lifted.