waist band; sash
A soft, informal sash traditionally worn by young men and children with yukata or everyday kimono; wider and more casual than a formal obi.
You can wear a yukata comfortably by pairing it with a heko obi.
The term 兵児帯 (へこおび) is thought to derive from 兵児 (へこ), an old word for a young boy or servant, combined with 帯 (おび, 'sash'), reflecting its historical association with young males' informal attire.