transitive & intransitive verb .
To make or become longer.
The Century Dictionary and Cyclopedia
To make long or longer; extend or elongate in space or in duration; protract or prolong: as, to lengthen a line; to lengthen life; to lengthen a vowel or syllable in pronunciation.
To grow long or longer; extend in length.
the GNU version of the Collaborative International Dictionary of English
intransitive verb.
To become longer.
transitive verb.
To extent in length; to make longer in extent or duration