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I call the "main mirror" the primary because it is the first mirror the light hits. The next mirror it hits is the "secondary" for the same reason, however, I don't have a problem if someone calls it the "diagonal" (providing they are talking about a Newtonian, or similar design) although I prefer to use secondary. If you have enough "in focus" you could then put in a "diagonal" (or "star diagonal") to bend the light path around, although technically, I guess it should be called the "tertiary" in order to be consistent. In the case of an SCT, I would also call this a "diagonal" as it achieves the same effect as in a Newtonian.
I think using the primary and secondary terms makes it obvious what you are talking about. So when you say "secondary" you mean the second mirror the light hits, regardless of Newtonian, SCT, Mak, or whatever type of design you are talking about. If you were talking about collimating a "reflecting" type of scope, you would say that, for example, the secondary must be in the centre of the primary, which is true, AFAIK, for all types (that have a secondary). If you were aligning the "diagonal" it could be assumed you are talking about a Newtonian type of design.
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