The term 'tightly sealed' can be described by the word 'HERMETIC', which is an 8 letter word. HERMETIC refers to something that is airtight and impermeable, meaning that no air or other substances can pass through it. This term is often used to describe containers, such as jars, that are designed to keep food fresh for a longer period of time. It is also used to describe objects that are designed to be waterproof and dustproof.