Lisbon

As a proper noun referring to a specific geographic location, this word is used exclusively to denote the capital of Portugal. It functions as a unique identifier and does not describe a category of things.

Because it is a proper noun representing a single entity, it is treated as an uncountable singular noun in grammatical practice. It does not take a plural form and is not used with indefinite articles like "a" or "an" unless modified by an adjective (e.g., "a sunny Lisbon").