(medicine) Initialism of rule out: used to indicate that a certain possible diagnosis should be considered, and appropriate tests performed to rule it out.

An "r" that is pronounced only because it is closely followed by another morpheme beginning with a vowel sound, as in water ice, in non-rhotic varieties of English.

(grammar) A referential expression, such as "John" or "the dog": one that, unlike pronouns and anaphora, independently refers to, i.e., picks out, an entity in the world.

plural of R-expression

(typography) A curved form of the letter r, found in some medieval and fraktur scripts: ⟨ ꝛ ⟩.

Initialism of reserves-to-production.

Shaped like the capital letter "R" or its mirror image

The book The C Programming Language, which described the first edition of the C language.

Initialism of kidnap and ransom.

(algebra) The monoid action of a ring R on an abelian group.