Refers to one's feelings or affections shifting to another person or thing; often implies inconstancy in love or interest.
His fickleness is so extreme that it's hard to be in a relationship with him.
I became anxious, suspecting that her heart had turned to someone else.
Compound of 心 (こころ, 'heart, mind') and 移り (うつり, the continuative form of 移る 'to move, shift'). The exact historical derivation is uncertain, but the word transparently means 'heart shifting'.