grat·i·tude [grat-i-tood, -tyood]
noun
the quality or feeling of being grateful or thankful: He expressed his gratitude to everyone on the staff.
That's your basic definition of gratitude. But it's much more than that.
Gratitude is a positive emotion. Gratitude is the link between Faith, Hope and Love. Gratitude is transmutable, or can changed from one nature, substance, form, or condition into another. Because of this whenever you give gratitude you always get something back. The more you give the more you get.
