a decision is a choice made between two or more options; it often involves weighing up the pros and cons of each option and deciding which is the best. It is usually used to refer to something of significance or importance; as such, decisions are often the result of careful consideration and come with ramifications.
In most cases, decision is used as an uncountable noun, meaning that it cannot be pluralized. It is typically preceded by an article ('the') when used in writing, especially in formal contexts.
1. She had to make a tough decision.
2. The decision made by the board of directors was unpopular.
3. They came to a decision after a lengthy discussion.
Note that a decision can also be used as a verb to refer to the act of deciding ('He decided to take a different route home'). Additionally, when used as a noun, the word decision (without any article) is often preceded by an adjective to indicate the kind of decision being discussed ('a wise decision,' 'a controversial decision,' etc.).