The tenor of a metaphor is the thing being described or compared. It is the subject the reader is meant to understand more fully through the image or comparison. In the metaphor "Time is a thief," time is the tenor, and thief is the vehicle, or the image used to carry the comparison.
Writers and critics use this term to separate the idea being explained from the image used to explain it.
Example
Her laugh was a window flung wide to morning.
In this metaphor, "her laugh" is the tenor, and "a window flung wide to morning" is the image that helps describe it.
Related Definitions
[n] the pitch range of the highest male voice
[n] pervading note of an utterance; "I could follow the general tenor of his argument"
[n] the adult male singing voice above baritone
[n] an adult male with a tenor voice
[adj] of or close in range to the highest natural adult male voice; "tenor voice"
[adj] (of a musical instrument) intermediate between alto and baritone or bass; "a tenor sax"