What it is
Tiny random differences in duration between adjacent vibration cycles, usually expressed as a percentage. Perfectly regular cycles do not exist, but a well-trained voice jitters very little; growing jitter means unstable fold vibration.
Where it comes from
Measure the cycle lengths one by one and compute a statistic of adjacent-cycle differences (such as the mean relative jitter).
How to read it
Healthy adult voices usually measure jitter below 1%; persistently elevated values hint at instability at the fold-closure or neurological level (fatigue, pathology and aging all push it up). Deliberate vibrato in singing is not jitter — vibrato is periodic, jitter is random.