An elastic loop used to tie hair; generally made of rubber and called a hair elastic/hair tie.
朝、ヘアゴムで髪をひとつにまとめた。
In the morning I tied my hair into one ponytail with a hair elastic.
ヘアゴムを腕にかけておく癖がある。
I have a habit of keeping a hair elastic on my wrist.
ヘアゴムが切れたので、新しいのを買ってきた。
My hair elastic broke, so I bought a new one.