What is emotion?

I looked up a definition of emotion: “A natural instinctive state of mind deriving from one's circumstances, mood, or relationships with others.” We often view emotion through facial expressions - a smile, a frown, a wrinkled brow, or maybe tears. What if we can’t see someone’s face; can we also feel emotion? I saw a woman and her partner arm in arm walking close. As they stopped, there was an embrace. I love the arm on the shoulder and the hand gripping the waste. To me there is so much emotion in this image without any facial expression. I can feel the warmth between them.