waist·line 
/ˈweɪstˌlaɪn/ noun
plural waist·lines /ˈweɪstˌlaɪn/ noun [count] 1 : the distance around the narrowest part of your waist
▪ The diet claims it will reduce your waistline in just four weeks. ▪ I'm watching my waistline. [=I'm trying not to eat too much so that I don't gain weight] 2 : the part of a piece of clothing that covers your waist
▪ The dress has a small waistline and a full skirt.
▪ The diet claims it will reduce your waistline in just four weeks. ▪ I'm watching my waistline. [=I'm trying not to eat too much so that I don't gain weight]
▪ The dress has a small waistline and a full skirt.
