clincher

noun
  1. 1

    That which clinches; that which makes something final or firm.

    โ€œThe clincher was that we couldn't wait any longer to leave, or it would get dark.โ€

  2. 2

    A tyre with a bead around the edge to attach to the rim of the wheel when inflated.

Source: https://en.wiktionary.org/wiki/clincher