Gratitude, thankfulness, gratefulness, or appreciation is a feeling, emotion or attitude in acknowledgment of a benefit that one has received or will receive. The experience of gratitude has historically been a focus of several world religions, and has been considered extensively by moral philosophers such as Adam Smith. The systematic study of gratitude within psychology only began around the year 2000, possibly because psychology has traditionally been focused more on understanding distress rather than understanding positive emotions. However, with the advent of the positive psychology movement, gratitude has become a mainstream focus of psychological research. The study of gratitude within psychology has focused on the understanding of the short term experience of the emotion of gratitude (state gratitude), individual differences in how frequently people feel gratitude (trait gratitude), and the relationship between these two aspects.

Gratitude is not the same as indebtedness. While both emotions occur following help, indebtedness occurs when a person perceives that they are under an obligation to make some repayment of compensation for the aid. The emotions lead to different actions; indebtedness can motivate the recipient of the aid to avoid the person who has helped them, whereas gratitude can motivate the recipient to seek out their benefactor and to improve their relationship with them.

Captain America is a fictional character, a superhero who appears in comic books published by Marvel Comics. The character first appeared in Captain America Comics #1 (cover-dated March 1941), from Marvel Comics' 1940s predecessor, Timely Comics, and was created by Joe Simon and Jack Kirby. As of 2007, an estimated 210 million copies of "Captain America" comic books had been sold in 75 countries. For nearly all of the character's publication history, Captain America was the alter ego of Steve Rogers, a frail young man who was enhanced to the peak of human perfection by an experimental serum in order to aid the United States war effort. Captain America wears a costume that bears an American flag motif, and is armed with an indestructible shield that can be thrown as a weapon.

An intentionally patriotic creation who was often depicted fighting the Axis powers of World War II, Captain America was Timely Comics' most popular character during the wartime period. After the war ended, the character's popularity waned and he disappeared by the 1950s aside from an ill-fated revival in 1953. Captain America was reintroduced during the Silver Age of comics when he was revived from suspended animation by the superhero team the Avengers in The Avengers #4 (March 1964). Since then, Captain America has often led the team, as well as starring in his own series.