When something is both delicious (om nom) and spectacular.
This word can be used in a broad number of situations (as shown in a couple of examples found below)
Ex. "This cheesecake is omnomnomtacular!"
Ex. "Bacon to cure this hangover? What an omnomnomtacular idea!"
Ex. "Oooohhh (insert attractive male/females name) is looking omnomnomtacular tonight!"