What is Gibraltar Is it an isthmus?

Gibraltar is neither an isthmus or a 'penninsular'... but it IS a peninsula! Gibraltar, which is a British Overseas Territory at the southern tip of the Iberian Peninsula is connected to the Spanish mainland by a 1.2 km (0.75 mi) wide (roughly from east to west) isthmus much of which forms a land border with Spain.

More information on the geography of Gibraltar can be found on the link provided.