Feedback is an essential concept of control theory. There are various, but similar, definitions for feedback.
The term feedback refers to a situation in which two (or more) dynamical systems are connected together such that each system influences the other and their dynamics are thus strongly coupled. [1]
So when we talk about feedback we are talking about how some characteristic of a dynamic system influences the evolution of that system. That is, feedback is reactive.
However, sometimes it is uneasy to set forth the definition of feedforward.
..., in some circumstances it is possible to measure a disturbance before it enters the system, and this information can then be used to take corrective action before the disturbance has influenced the system. ... This way of controlling a system is called feedforward. Feedforward is particularly useful in shaping the response to command signals because command signals are always available. [1]
The above definition of feedforward involves disturbance rejection or command signal shaping. But it would be an inappropriate place to introduce the concepts of disturbance rejection and command signal shaping. Nevertheless, [1] provides a good example of comparing feedback with feedforward:
In economics, feedback and feedforward are analogous to a market-based economy versus a planned economy.
As far as my knowledge is concerned, the most elegant definition of feedback and feedforward is:
If the control portion introduces a loop to the dynamic system, we can call this feedback (control); otherwise, it is called feedforward (control).
Loop, in effect, is the core of feedback. This makes determining feedback according to the loop quite reasonable.
To be continued...
[1] Karl Astrom and Richard Murray, Feedback Systems: An Introduction for Scientists and Engineers, 2012.