Gas/ɡæs/ Definition 1 of 6Pro Tip 1/3Intangible substance Understand that 'gas' refers to something you can't hold like solids. The helium gas inside balloons is lighter than air. Pro Tip 2/3Invisible but real Remember gases may be invisible but they can have effects, like oxygen we breathe. Gas leaks can be dangerous even though you can't see them. Pro Tip 3/3Environmental impact Some gases can contribute to environmental problems like global warming. Factories can emit harmful gases into the atmosphere.