What it is
The depth of pitch deviation above and below the center note, measured in cents — 100 cents equal one semitone. It measures how wide the wobble is.
Where it comes from
Measure the peak offsets of the F0 undulation and take the mean or peak amplitude (± cents). Together with rate, this fully describes the vibrato.
How to read it
Classical singing commonly shows ±25–50 cents up to ±100 cents (± a semitone); extent is more trainable than rate. Too narrow sounds like straight tone, too wide like a wobble; pop styles often deliberately use a narrow extent or straight tone.