From the Ming-dynasty writer Wu Hai's 《送傅德谦还临川序》: 读书有得，冥然感于中，心领神会 - when reading yields insight, one feels it inwardly and grasps it in heart and spirit. Originally about reading; now about understanding signals from another person.
usage
Describes wordless understanding between people (eye contact, hints, context) or deep comprehension of a text/teaching. Often paired with 一 + action: 一个眼神就心领神会 (understood it from a single glance).
