Emieno, You asked for a "definition" of the "concept" called emptiness. Where do we find definitions--in dictionaries. Now, what dictionary to use--great question--little agreement among philosophers. I use the Webster Unabridged Dictionary. Here we find the various definitions for the concept emptiness:
1. a state of being empty; a state containing nothing; absence of matter
2. void space; vacuity; vacuum
3. want of solidity or substance
4. want of intellect or knowledge; lack of sense
Now your thread asks, how can one "feel the emptiness". Then, what does it mean to "feel", e.g., how is this concept defined, as it may relate to emptiness. Again to Webster:
to feel--to be aware of the condition of being, as I feel sad.
Now, let us combine the two definitions (feel & emptiness) and we see that a "feeling of emptiness" can be defined as ---to be aware of the condition of having absence of matter. Many examples exist, consider if you have lost someone you love (absence of life), then you clearly understand what it means to be aware of absence of matter, you are aware of what it means to have a "feeling of emptiness". In short, you can never know the feeling of emptiness unless you are aware.