The process by which matter changes directly from a solid to a gas is called sublimation.
Literal
Matter [subject-が] directly solid [from-から] gas [target-に] change process [object-を] sublimation [quotative-という].
A definitional sentence using ~という (is called). The relative clause 物質が直接固体から気体に変わる modifies 過程 (process). 昇華 (しょうか) means sublimation — a technical term. Good example of how Japanese packs complex information into pre-noun modifiers.