An HM that contains Cut.You can only use outside battle when you get a certain gym badge.
Kanto R/B/Y/FR/LG: S.S. Anne (Recieve from the captain.)
Johto G/S/C: Ilex Forest (Obtain after chasing 2 Farfetch'd)
Hoenn R/S/E: Rustboro City (Recieve from the Cutter)
Sinnoh D/P/Pt: Eterna City (Recieve from Cynthia if you try to walk to the shrine with Dialga (Diamond) Palkia (Pearl).