The word "apply" means to make a formal request or to put something into action. It can refer to using a concept in a practical situation, submitting an application for something, or ensuring that something is relevant in a particular context. For example, when someone applies for a job, they are submitting their credentials to be considered for that position. Similarly, if you apply a theory in science, you are using that theory to explain or test something in the real world.
The word "apply" comes from the Latin word "applicare," meaning "to attach to" or "to join to." This Latin term is composed of "ad-" meaning "to" and "plicare" meaning "to fold." The concept of applying something can be understood as folding it into a new context or situation.