h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

A tick is only one second if you are looking at a quartz analogue clock or watch. These tick once per second because that's how often the second hand moves. (No second hand = no tick.) An old-fashioned clockwork watch can tick more than once per second, depending on the mechanism, and they don't 'tick-tock' the way a clock does, so a tick isn't half a tick-tock.

The length of time taken by the 'tick-tock' of an old-fashioned clock also varies with the kind of clock. In some cases it depended on the length of the pendulum swing. In others it depended on the clock mechanism. I've never heard of anybody keeping time by counting 'tick-tock' as the article suggests, simply because of this variability.

Having said all that, it's a very interesting article. I guess I'd just have to go with definition 3.