Pedaling not connecting right away I ride a rather usual Merida hard-tail mountain bike.
Recently I've come to notice that when I start pedaling (bike already in motion), it can take up to as much as quarter of a turn for the pedals to click and the chain to start pulling the bike again.
What could cause this and more importantly, how can this be fixed? Is this merely a problem of chain tension?
The parts that Blam refers to are known as the pawls and the ratchet inside the freehub. Slipping can happen for a number of reasons, including worn ratchet, worn pawls, weak springs, or excessive accumulation of grease &amp; grime within the freehub.

http://dirtmountainbike.com/features/work-freehub-body.html has a thorough explanation of the different freehub types.

Depending on the hub you have, a repair consists of either cleaning, rebuilding or replacing the freehub. Some freehubs are rebuildable, others are not and must be replaced as a unit. That said, it's often possible to extend the life of a non-rebuildable freehub by flushing out the old grease &amp; gunk and regreasing.

The mechanic at your local bike shop can almost certainly take care of this for you. If you're more of the do-it-yourself type, http://www.parktool.com/blog/repair-help/freehub-service has step-by-step instructions for several different freehub types.