Typically the term "relief valve" means one that opens to to vent excess pressure. An air release valve contains a float that will open a valve to vent air out of a fluid filled system. It will vent gas but not liquid. However, it is also possible that the terms are used interchangeably by some.
Most likely, but that is only a guess as to what some third party meant. There is no legal definition of these terms. The meaning is often made clear by context or by asking for clarification from the person who used the term. Many people are very imprecise in their use of language, particularly the inexperienced. Relief and release sound very similar. I saw two instances today on this site where posters had used the wrong word, which sounded similar to the correct word.
An air release valve will be one set up to separate gas from liquid, typically using a float system. These do not necessarily open at any set pressure or pressure differential, only by the buoyancy of the float.
An air relief valve will typically be set up as a valve set to open at a particular pressure, and designed to deal with the flow of gas (rather than primarily liquid).
Relief valves are for pressure, release valves are for phase separation. Note that neither of these is a precision device for control. If you need good pressure control or really good phase separation, there are better ways to do it.
