A restless state of mind that wanders away from an object while observing it is a mental state called restlessness (uddhacca). It flies away from the object. Due to restlessness the mind cannot remain very long with an object but often wanders elsewhere.
Mahasi SayadawRelated topics
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Momentary concentration that focuses on mental or physical objects from moment to moment is called nondistraction (avikkhepa), a mental state that is the opposite of restlessness. When momentary concentration becomes strong, the mind seems to penetrate its object every time an object is noted.
Mahasi Sayadaw