In computing, reactive programming is a programming paradigm oriented around data flows and the propagation of change. This means that it should be possible to express static or dynamic data flows with ease in the programming languages used, and that the underlying execution model will automatically propagate changes through the data flow.
For example, in an imperative programming setting, a := b + c would mean that a is being assigned the result of b + c in the instant the expression is evaluated. Later, the values of b and c can be changed with no effect on the value of a. In reactive programming, the value of a would be automatically updated based on the new values.
A modern spreadsheet program is an example of reactive programming. Spreadsheet cells can contain literal values, or formulas such as =B1+C1 that are evaluated based on other cells. Whenever the value of the other cells change, the value of the formula is automatically updated.