Oblique (a grammatical relation)

The term oblique is often used to refer to the grammatical relation of arguments or adjuncts that are coded in some special way (by means of an adposition or a semantic case), contrasting with core arguments, which are generally coded without case-marking or with grammatical case-marking.

Examples
In the sentence On Monday Maria sent a letter to her mother by e-mail, the three prepositional phrases on Monday, to her mother, and by e-mail would be considered obliques by many linguists. Note that on Monday and by e-mail are adjuncts, while to her mother is an argument; typically the term oblique is neutral with respect to the adjunct-argument distinction and is defined by the coding patterns.

Polysemy

 * oblique case refers to non-nominative case.

Origin
The term oblique must have been inspired by oblique case, though its meanings diverges considerably. It is a technical term in Relational Grammar, and it may well be that it spread to linguistics from there.

Reference

 * Thompson, Sandra A. 1997. "Discourse motivations for the core-oblique distinction as a language universal." In Kamio, Akio (ed.) Directions in functional linguistics. Amsterdam: Benjamins, 59-82.