When is a tax a tax?

Nick Clegg, writing in the Guardian, is obviously stung by the claim that the Lib Dems want to bring in a tax on dogs.

"Apparently, the newspaper went on, a vote for me would usher in all sorts of additional evils such as - wait for it - a "tax on pets". Apart from the relatively minor fact that it is not Lib Dem policy to tax pets, but simply to introduce a dog registration scheme as demanded by countless pet owners since dog licensing was abandoned in 1987, I was left nonplussed."

Okay, it's not a tax, but everyone with a dog will be forced to pay it, it sounds like a tax to me.