Which of the following best describes a constant?

A constant is defined as a fixed value that does not change. In mathematics, constants are numbers that remain the same regardless of the conditions around them. This is pivotal in equations, expressions, and functions where certain values need to be stable in contrast to variables, which can vary.

For instance, in the equation y = 2x + 3, the number 3 is a constant because its value remains fixed while x can take on different values. Understanding the role of constants is essential for grasping how mathematical expressions and equations are structured.

Choosing other options may lead to confusion because a number that can change refers to variables, a letter representing an unknown quantity typically denotes variables as well, and a mathematical sentence is a broader term that does not specifically define a constant. Therefore, selecting the correct answer highlights the foundational understanding of constants in mathematical contexts.