Ghost-type Pokémon are weak to Dark-type and Ghost-type moves in the mainline Pokémon games.

Core weaknesses

  • Dark-type moves (like Crunch, Bite, Knock Off) deal super-effective (2×) damage to pure Ghost-type Pokémon.
  • Ghost-type moves are also super-effective (2×) against Ghost-type Pokémon, so they are both strong and weak to their own type.

Extra battle notes

  • Ghost-types are immune to Normal- and Fighting-type moves, so you generally cannot hit them with those attacks unless specific abilities or moves override that.
  • Some dual-typed Ghost Pokémon can end up with additional weaknesses or resistances, but Dark and Ghost remain the baseline weaknesses for the Ghost type itself.

TL;DR: When asking “what is Ghost type weak to” , the answer is Dark and Ghost moves.

Information gathered from public forums or data available on the internet and portrayed here.