The battle was converted into a strategic defeat because it was not followed up with more British success. The British withdrew back to Sicily. That's a strategic defeat and the tactical victory at Maida was wasted.

Tactics is the art of fighting battles; strategy is using those battles to win a campaign or the war. The tactical success at Maida was not used to follow up and win strategically.

It's a very simple concept and if you look at a good military dictionary I'm sure you'll understand it.