A constant can be expressed using multiple equivalent units like – SI, natural units, atomic mass unit, CGS units etc. Some physical constants are redefined by fixing the SI values. But this certainly increases the uncertainty in the value of physical constants. The C++ const keyword is used to specify that the value of a variable cannot be changed. In this tutorial, you will learn about C++ constants with the help of examples. Constant definition: not changing or varying; uniform; regular; invariable.. See examples of CONSTANT used in a sentence. Learn the meaning of constant as an adjective and a noun in English, with synonyms, antonyms, and usage examples. Find out how constant is used in mathematics, science, and everyday language.