It is a "substantive" - an adjective that plays role of a noun. There are many such words, some of them exist in parallel with corresponding adjectives (борзая, гнедой, верховой, дозорный, военный, ванная, кривая, плясовая) - so called "motivated substantives". Others nowadays exist only as nouns (вселенная, городничий, запятая, мостовая, портной, родословная). They inflect as adjectives but are used like nouns.

In English situation is very much the same but as soon as there are not much inflections in English, nobody pays attention.