The adjective essential is used to describe things that are absolutely necessary for a particular purpose. For example, discipline is essential in an army. This all-important descriptor is related to the noun essence, which is defined as “the basic, real, and invariable nature of a thing.” Indeed, sometimes essential suggests that which is in the very essence of constitution of a thing: Oxygen and hydrogen are essential in water. The synonym indispensable is a strong synonym for essential: it is used to describe people or things that are absolutely necessary or requisite—in other words, people or things that cannot be done without: She was an indispensable member of the staff.

The adjective secret is used to describe things that are either done without the knowledge of others, such as secret negotiations, or that are working to escape notice or observation, such as secret police. It’s also used to describe things that are secluded or sheltered, such as a secret hiding place—and things that are known by only a few, such as a secret password. The synonym clandestine is used to describe operations, meetings, and programs that are done in or executed with concealment, especially for the purposes of subversion or deception. If you’re looking to describe the activities of international spies or stealth programs run by the military or a government, clandestine is the perfect choice.

The verb drop suggests an abrupt descent or downward trajectory. It can be used of items that are falling in a literal sense, such as rain from the sky or acorns from a tree, or of immaterial things—such as prices or temperatures. The synonym plummet suggests an even more rapid and steep descent, as if being pulled down by a weight. There's a reason for the extra weight: the word plummet (noun) originally referred to a piece of lead or some other weight attached to a line, used for determining the perpendicularity or depth of something. So, when prices or temperatures plummet, they decrease sharply and dramatically, as if being pulled down by something heavy.