sweet·heart 
/ˈswi:tˌhɑɚt/ noun
plural sweet·hearts /ˈswi:tˌhɑɚt/ noun 1 a [count] somewhat old-fashioned : a person you love very much
▪ childhood/college sweethearts
b — used to address someone you love ▪ How was your day, sweetheart? [=darling, dear] ▪ Sweetheart, what's the matter? 2 [count] : a kind or helpful person : someone who is very nice
▪ Be a sweetheart and help your grandmother. ▪ She is such a little sweetheart.
▪ childhood/college sweethearts
b — used to address someone you love ▪ How was your day, sweetheart? [=darling, dear] ▪ Sweetheart, what's the matter?
▪ Be a sweetheart and help your grandmother. ▪ She is such a little sweetheart.
