I have seen this term used often and have a vague idea as to what it means, but since I have not seen this term used in other fields I thought I would ask here. Is any response from the Patent Office considered an "Office Action"?
In the U.S. the term Office Action is typically used to describe a notice a patent applicant receives from a patent examiner, informing the applicant that the examiner has decided to reject some or all of the claims in an application and explaining the reasons why (e.g. a claim A is anticipated by reference X, claim b is obvious in view of references X and Y, claim C is ambiguous, and claim D is allowed).
From Wikipedia: An office action may be "final" or "non-final". In a non-final office action, the applicant is entitled to reply and request reconsideration or further examination, with or without making an amendment. In a final office action, the applicant has two options for reply. In the first option, the applicant may appeal rejection of claims to the Board of Patent Appeals and Interferences [now called the Patent Trials and Appeals Board]. Otherwise, the applicant may file an amendment which complies with the requirements set forth in the office action. Reply to a final rejection must include cancellation of, or appeal from the rejection of, each rejected claim. If any claim stands allowed, the reply to a final rejection must comply with any requirements or objections as to form. Replies to final office actions must be in accordance with 37 C.F.R. 1.113-1.114
Note: I'm a USA practitioner, so what I say goes for the USPTO.
There isn't a strict definition about what constitutes an "Office action". As generally used, the term generally encompasses any mailing from a patent examiner; it usually excludes decisions on petitions, communications from the Office of Patent Application Processing (the people who handle the applications before they get to the patent examiners), and communications in the appeals process (for example, decisions by the PTAB, or Examiner's Answers). On the other hand, because it's not a strictly defined term, you can't really mess anything up by using it incorrectly.
The most common types of Office actions are non-final and final rejections, notices of allowance, requirements for election or restriction, requirements for information, and Ex Parte Quayle actions. These are all notifications that the Office is taking some kind of specific action (ie, allowing a claim, rejecting a claim, objecting to something in the application, or requiring the applicant to do something) that is directly related to the patentability of a specific claim. Hence the term "Office action".
In general only communications from the Examiner or Examining Division which raise objections to the patentability of the claims that are pending are called Office Actions. Be aware that this is not an official term. The official term differs from jurisdiction to jurisdiction and may be called 'Communication pursuant to Art. 94(3)' [EPO], 'Notice of Rejection' [JPO] and so on.