The adjective “prior” refers to something that occurred earlier in time or with a higher order of importance.
I have a prior commitment, so I cannot have lunch with you today, but thanks for asking.
Corporate writing tends to use “prior to” as a preposition to convey the thought of “before,” which is preferred. Another no-no is to use “prior” to mean “beforehand” or “earlier.”
Prior to commencing production, the well casing is cemented in place.
Vacation days must be approved by your supervisor at least 24 hours prior.
Previous is also used to mean “prior,” but it has a connotation of “preceding.” Many times you can omit the word “previous” because it is redundant.
In our previous discussion, we decided not to perforate that interval.
(Of course the discussion we had in the past preceded this one – redundant.)
In the previous meeting (the meeting preceding the one we are now talking about), we decided not to perforate that interval.
From whence did the word “prior” come?
In the middle ages, the prior was the superior of the monastery or other religious house, whereas the prioress was the chief ranking nun of the convent. Thus “prior” came to mean higher ranking, or person with higher priority.