B2[C or U]a substance in a form like air that is neither solid nor liquid: Oxygen, hydrogen and nitrogen are all gases.poisonous/inflammable/toxic gasThe state of matterA2[C or U]a substance in a form like air that is used as a fuel for heating and cooking: Do you prefer cooking with electricity or gas?UKA leak in the gas mains(= pipes) caused a major explosion.a gas-fired power stationFuelsPetroleum products especially when used as fuel›[U]informala substance in a form like air used for medical purposes to prevent people feeling pain or being conscious during an operation: I had/was given gas when I got my wisdom teeth out.SurgeryMedicine in different forms›[U]mainly US forwindnoun: Beer gives me gas.Digestive disorders