This is from the AVEN wiki and I think it is good enough.
Put as simply as possible, it is a distinct fondness or affection toward someone that differs from what you would feel toward friends, family or people you admire. It may be characterized by a unique, almost surreal anxious-euphoria when sensing or thinking about this person and is distinguishable from hero worship. It typically involves butterflies in the stomach, heart fluttering or “melting” when interacting with them, some obsessiveness, all over warm and fuzzy feeling, and being swept into a dreamy state of mind, but experiences may vary depending on the individual and intensity of the case. Desires such as wanting to bond or be physically close with them are extremely common when experiencing romantic attraction