A nebula is a large cloud of interstellar gas or dust. In nontechnical use, the word nebula also refers to a galaxy other than the Milky Way.

// We were eventually able to see the nebula through the telescope.

Liminal is a formal word most often used to describe an intermediate state, phase, or condition. It can also describe something that is barely perceptible or barely capable of eliciting a response.

// The essay presents an image of the border region as a liminal zone where one culture blends into another.

Callow is a synonym of immature used to describe someone, especially a young person, who does not have much experience and does not know how to behave like an adult. Like the word immature, callow is often used disapprovingly.

// The novel’s plot involves a callow youth who eventually learns the value of hard work and self-reliance.

To fraternize with someone is to be friendly with them or to spend time with them in a friendly way. Fraternize is often, though not always, used in situations where such friendly behavior is considered wrong or improper, as in “fraternizing with the enemy.”

// The boss warned that fraternizing with the junior employees could be a risky career move for a manager.

Something described as chockablock is very full or tightly packed.

// Their mantel is chockablock with knickknacks collected from their travels to all fifty states.

Someone described as ambivalent has or displays very different feelings (such as love and hate) about someone or something at the same time.

// Aya was ambivalent about starting her first year away at college—excited for the new opportunities that awaited but sad to leave her friends and family back home.

Memento is a synonym of souvenir; it refers to something that is kept as a reminder of a person, place, or thing.

// The box on the shelf in her closet is filled with mementos of her basketball career—awards, newspaper clippings, team photographs, and her old uniform.

close in time; about to occur

A gridiron is a football field. The word gridiron is also sometimes used to refer to a grate for broiling food (as in “put the steaks on the gridiron”) or something consisting of or covered with a network (as in “a gridiron of streets and avenues”).

// The book recounts the quarterback’s most glorious moments on the gridiron.