In case; if; -- used to introduce the first or two or more alternative clauses, the other or others being connected by or, or by or whether. When the second of two alternatives is the simple negative of the first it is sometimes only indicated by the particle not or no after the correlative, and sometimes it is omitted entirely as being distinctly implied in the whether of the first.
source: Webster 1913
pronoun
Which (of two); which one (of two); -- used interrogatively and relatively.
