An adjective phrase is a phrase whose Head word is an adjective. As with other phrases adjective phrases can consist of only one word (the Head) or of more than one word.
Note that the National Curriculum stipulates that phrases should have at least two words, though it concedes in the Glossary entry for noun phrases that "Some grammarians recognise one-word phrases."
Here are some examples of adjective phrases in sentences. The phrases are marked in square brackets and the Head is highlighted.