Sorry about that, folks. I was tired and a bit in a hurry when I posted without really looking at the sample sentence all that hard. It is not the conditional mood at all, as you have all pointed out. Just wish I'd caught it sooner.

Well, the voice of the example is passive. As others have suggested it seems to be a kind of historical future. A future tense expressed within the context of the past. The mood is just plain indicative as there is nothing in doubt about the event (the definition) occurring.

Again sorry for the noise caused by my earlier confusion.

Ceci n'est pas un seing.