Is quam an adjective or adverb?

Question

Keller's Learn to Read Latin says quam is a relative adjective or an interrogative adjective:

The quam of the adverb quam ob rem may be either a relative adjective [see §86)-“on account of which thing," "therefore”—or an interrogative adjective [see §88), “on account of which thing,” “why.” Quam ob rem may be written as a single word (quamobrem).

Oxford Latin Dictionary says quam is an interr. and rel. adv.:

quam interr. and rel. adv. [acc. sg. f. of *quo- (QVIS1); = Osc. pan]

Which one is correct?

By the way, rem doesn't appear in the dictionary. What does it mean?

Answer 1

The first thing to understand is that quam ob rem is a fossilized prepositional phrase.

The preposition ob takes an accusative object and means “on account of.”

The object of ob is rem (accusative singular of res), which means “matter, issue, state of affairs.”

Quam is a form of the pronominal adjective qui; it is accusative feminine singular because it modifies rem.

Much like the English pronoun “which,” qui can either ask a question (which matter would you like to discuss?) or refer to something previously stated (which matter is the topic of our discussion).

These functions of qui lead naturally to the two functions of quam ob rem as a whole. If the quam is interrogative, the phrase as a whole introduces a question: “on account of what matter?” If the quam refers back to something else, the phrase draws an inference: “on account of which matter”, i.e., therefore.