drawstring

/ˈdrɔːstrɪŋ/
noun
  1. A string or cord that is threaded through a casing or hem and used to tighten or close an opening, such as on clothing, bags, or curtains.
    • The backpack closes with a simple drawstring at the top.
    • He replaced the broken drawstring in his sweatpants with a new one.
    • She pulled the drawstring on her hood to keep the wind out.