Noun
training pants pl (plural only)
Undergarments worn by incontinent people, typically infants, to aid in toilet training during the transition between diapers and underpants.
He called for further studies to be done on the topic. citation Training pants main Manufacturers have designed "training pants" which bridge the gap between baby diapers and normal underwear during the toilet training process. Source: Internet
The product range has more recently been extended into children's toilet training phase with the introduction of training pants and pant diapers, which are now undergarments. Source: Internet