altitudo

Altitudo, altitudinis is third declension feminine, formed from altus with the abstract suffix -tudo that also gives magnitudo, longitudo and latitudo. The stem is altitudin-, so the nominative tells you nothing useful and you have to learn the genitive.

The trap is inherited from the adjective. Altus means both high and deep, so altitudo is height or depth according to what is being measured, and only context decides. Caesar uses it constantly of the depth of a river and the height of a rampart, sometimes on the same page. English altitude kept only the upward half, which loses the better joke.