What is gratitude (in simple words)?

Today everyone is talking about gratitude and there are many self-help books that shed light on the benefits of practicing it. So what is it, in plain and simple words?  

Definition

Gratitude is an emotion that overwhelms you when you realize that someone or something has done you a favor by benefiting you, and you were not even entitled to receive that benefit.

Feeling appreciative of someone (God, people) or something (universe, material possessions, health, money, etc)

Gratitude revolves around these two factors:

1) Recognizing a benefit
2) Acknowledgment or appreciation for the source/benefactor (the person or thing that brings that benefit)

What gratitude isn't

Most people mistakenly think that gratitude is all about only recognizing a gift and feeling happy for it ( for example, winning a lottery and celebrating it), but the fact is, if it is not directed toward the source/benefactor that brings that benefit (lottery), gratitude has not happened. Appreciation and acknowledgment for the source is an essential part of it.

Is gratitude the same thing as positive thinking?

Another misconception about gratitude is that it is merely a form of positive thinking. Robert Emmons, who has done an extensive research in the subject, sheds light on some of the myths about  gratitude. Check out this video:

Understand gratitude with an example

If I bring you a glass of water, you will feel grateful to me, why? Because you realize it was outside of you and beyond your control. I (source/benefactor) could have refused to bring you water or maybe spilled it all over your face in anger. So naturally, you will feel gifted or will realize that I have done you a favor by bringing you water. 

Another example: 

Availability of water alone should create the feeling of gratitude in us as it is available in the world due to complex physical and natural laws that are certainly beyond our spiritual/physical capability or control. We, as humans, had no role in creating water. It was created by something outside of us - our Lord who creates everything. Imagine this gift (water) gone for two days straight and you will create an intense feeling of gratefulness overwhelming you right now. 

In addition to recognition of benefits, the feeling of gratitude can also occur when we realize that something bad that could have happened, didn't happen in reality.

Explanation: Imagine why you would feel intense gratitude to God or your luck when, let's say, a speeding truck nearly hit you and crashed into a wall without hurting you? Because you are away of the fact that the truck could have crashed into you, and you were not entitled to be saved. So, naturally you will be forced to thank God deeply for protection (a benefit and relief from a loss) 
Here your mind quickly assess a negative outcome that a truck crash would create and when that possible negative outcome (serious injuries or even death) does not happen, you feel a huge sigh of relief (a gift from something outside of yourself - God) and thankfulness overwhelms you and start feeling intense gratitude.