..."the relation of something to the matter in hand"
..."a term used to describe how pertinent, connected, or applicable something is to a given matter"
..."having a bearing on or connection with the subject at issue"
Relevance is measured by impact. Something is relevant to something else if it has an effect on it - if it has a bearing, a connection, an application to it.
So it's an active concept. You can appear relevant in any situation, but unless you are actually engaging with that 'matter in hand', and therefore having some kind of an effect on it, then you're not actually relevant at all.
But you can't engage with something, or someone, without first understanding it (or them). 1 Chronicles 12 recounts the story of the army that assembled around David following his banishment from Saul. These were the men who would help protect Israel's anointed king and would see him take eventually take the throne and so usher in the nation's 'golden-age'. Among them were counted the men of Issachar:
"men of Issachar who understood the times and knew what Israel should do..."
1 Chron 12:32
This is relevance in action. The men of Issachar recognised that David was the future and they needed to invest in him. They saw the way society was moving (under God's direction) and sought to engage and involve themselves with it, rather than sticking to the status quo (Saul). What's more, because they understood what was going on they 'knew what Israel should do'. Their relevance had a bearing a bearing on their actions.
The 21st Century church needs more men of Issachar. We need to understand our times, see a need, and go an meet it effectively. We need to relate to the matter in hand - see slavation reign in a decaying world.
The need is obvious - Jesus. And just as David's rule was announced before it came into being, so the Bible announces that all men will ultimately come to confess Jesus as Lord. But we're still not being that effective. The problem, I think, is often our warped sense of relevance. We think that if we look right, act right, talk right a.k.a. be "cool" then eventually everyone will come to embrace our faith because we're really cool, and not old and boring, and actually just like everyone else but with a little less sex and violence (but still really really cool). But the problem is, as far as my experience goes anyway, this just doesn't work! (because it's a Gospel based on ourselves and not on Jesus.) We might think we're being relevant, but if we're not actually positively affecting the thing to which we're trying to relate, if we're not understanding and doing (and doing well), then we're not actually relevant. Period. No matter how many Ramones t-shirts you own, or how skinny your jeans are.
Maybe, then, we haven't quite understood the times as well as we think we have? I guess the proof will be in the pudding. The men of Issachar witnessed David take control of the kindgom he was promised, will we be able to say the same about our generation? I hope so!