It's basically at what pressure/temperature combo a material is solid/liquid/gas. Water is unusual in that it's solid/liquid boundary (dotted green line) is angled so that higher pressure means it's more likely to be a liquid rather than a solid (it's more common for higher pressure to tend a material towards a solid state, solid green line). I think Germanium also shares this property, but I could have got the wrong element.
Also, an interesting concept that might be useful is superfluidity. Such as liquid helium that can "flow uphill".