A singlet is a sleeveless shirt worn as underwear or as a regular shirt.
A singlet is comfortable and casual, often worn in warm weather or for sports.
Examples:
I wear a singlet when it's hot outside.
He wore a white singlet to the gym.
The singlet is very comfortable.
I like to wear a singlet to bed in the summer.
I have a singlet in every color of the rainbow.
She wore a singlet to play basketball.