agradecimiento
/aɡɾadeθiˈmjento/ · B2 · noun · m
gratitude
agradecer (to thank) + -imiento (-ment) = thank-ment, gratitude
Agradecer comes from Latin gratus (pleasing, thankful), the same root as English grateful and gratitude. The suffix -miento turns a verb into a noun, much like -ment does in English.