The connotative meaning of love is someone having a great emotions towards someone. It is generally defined as being protective over them and putting their wishes before your own.
The scientific definition is when your brain releases a neurochemical which induces a flare in the limbic system of your brain. The limbic system is the system that controls all of your emotions (accept the fight or flight one). When the chemical is released, it makes you believe a mate is more fertile, resulting in reproduction. It is what makes humans the most dominant species on earth.