reverie
/ˈrɛv(ə)ri/
noun
A state of being pleasantly lost in one's thoughts; a daydream.
mass noun ‘I slipped into reverie’
Origin
Early 17th century from obsolete French resverie, from Old French reverie ‘rejoicing, revelry’, from rever ‘be delirious’, of unknown ultimate origin.
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Our memories of words past become reveries as they retreat into the distance. Some are remembered as light daydreams, others, rarely (we hope), as dark nightmares.