Epinoia derives from epi (upon/over) + nous (mind). It means a thought, design, or intention — the internal concept lying behind an external act. It appears only once in the NT (Acts 8:22), in Peter's withering indictment of Simon Magus.
Peter tells Simon Magus that "the thought [epinoia] of your heart" must be repented of. The word cuts deeper than the act: it is the thought, the interior motivation, the design that produced the sin of simony. This is the consistent prophetic critique: God does not see as humans see — He looks at the heart (1 Samuel 16:7). Jesus's Sermon on the Mount internalizes the law exactly here: anger before the act, lust before the touch, the epinoia before the deed. The gospel offers not just forgiveness of acts but transformation of the inward epinoia — the renewal of the mind (Romans 12:2).