"Liquid hydrogen" refers to hydrogen in its liquid state, which means it has been cooled down to a very low temperature where it becomes a liquid. Full definition
The temperature is very cold, and the surface of this gigantic planet is an ocean of liquid hydrogen that could be as much as 10,000 miles deep. (science.howstuffworks.com)
Consider this: One tablespoon of liquid hydrogen fuel — a mix of deuterium and tritium — would produce the same energy as 28 tons of coal. (fortune.com)
NASA can use existing capabilities and lower development costs by using liquid hydrogen and liquid oxygen (lox) for both the core and upper stages. (aerospaceguide.net)