noun
neckband; lanyard
A cord or band worn around the neck, often for holding an ID, whistle, or small item, or as an ornamental accessory.
社員証を首ひもで首から下げている。
I wear my employee ID around my neck on a lanyard.
笛の首ひもが切れてしまった。
The whistle's neck cord broke.
ストラップ is a broader term for any strap, including wrist straps and phone straps, while 首ひも specifically refers to a cord worn around the neck.
Compound of 首 (くび, 'neck') and ひも ('cord, string').