R

Recursion

The property of a system that permits the embedding of an element of the system within itself (e.g., in computer programs, a loop)

Reductionism

The position that all objects and events can be reduced to the lawful behavior of the elements of which they are constructed

Reference

What a word refers to, or denotes, as opposed to the meaning of the word. For example, the word Joe has no particular meaning; it merely refers to a person of that name (see sense)

Rehearsing

The repetition, either aloud or to oneself, of an item one wishes to remember temporarily; rehearsing keeps the item in short-term memory as long as it is continued

RNA

Ribonucleic acid, an essential component of  all cells, which functions in combination with DNA (see DNA)

Rube Goldberg contraption

An extremely complex machine consisting of common place  items organized in a very imaginative and surprising fashion in order to accomplish an absurdly simple task. Rube Goldberg was a cartoonist who drew such devices in the 1940s and 1950s