previous and prior imply existing or occurring earlier, but prior often adds an implication of greater importance.

PRIOR definition: 1. existing or happening before something else, or before a particular time: 2. before a…. Learn more.

Usage Note: Though prior usually modifies a noun that comes after it, as in prior approval, it sometimes modifies a noun for a unit of time which precedes it, as in five years prior.

PRIOR definition: preceding in time or in order; earlier or former; previous. See examples of prior used in a sentence.

2 days ago · This adjective has no positive form; rather, it serves as the comparative (prior) and superlative (prīmus) of the preposition prae. (Compare the preposition post, with comparative …

A prior claim or duty is more important than other claims or duties and needs to be dealt with first. The firm I wanted to use had prior commitments.

There are six meanings listed in OED's entry for the noun prior, two of which are labelled obsolete. See ‘Meaning & use’ for definitions, usage, and quotation evidence.

pri•or1 (prī′ ər), adj. preceding in time or in order; earlier or former; previous: A prior agreement prevents me from accepting this. preceding in importance or privilege. Idioms prior to, preceding; before: Prior …

Prior definition: Preceding in time or order.

OED's earliest evidence for prior is from 1607, in the writing of Barnabe Barnes, poet and playwright. It is also recorded as a noun from the Old English period (pre-1150).