"Liquid methane" refers to methane, which is a type of gas, that has been cooled down to a very low temperature to the point where it turns into a liquid form. Full definition
We have since found many bodies of liquid methane of varying sizes; for some reason, they mostly inhabit the high northern latitudes. (scientificamerican.com)
What's more, no one knows whether liquid methane can take the place of water for supporting life, he says. (sciencenews.org)
This world possesses rivers and lakes of liquid methane — though even the best telescopes can't see them through the moon's thick, nitrogen - rich atmosphere. (discovermagazine.com)