sweetheart is a word in English whose literal translation into Portuguese is "sweet heart" and can mean beloved, love. sweetheart it can be a noun or adjective and consists of a compliment or a affectionate way to treat someone.
Most of the time, the term sweetheart is used between two people who are in a loving relationship. It is a synonym for boyfriend/girlfriend, dear, etc. In English, it is also common for couples to call sweetie each other, a word meaning "sweetie".
She is the most important person in my life, she is my sweetheart! - She is the most important person in my life, she is my beloved!
Despite being a term of endearment used most often by a couple, sweetheart it can also refer to a friendly or generous person, or to some person or object that arouses loyal affection.
The word sweetheart it's quite common in the music business. An example of this is the song Sweetheart Like You (Loved Like You) by Bob Dylan or Sweethearts Together (Boyfriends Together) by the Rolling Stones.