In essence, when something is so awesome that it cannot be described by simple words such as 'sweet', 'cool', the root word 'radd' or the ending of 'spectacular'.
Originally taken from the To Write Love On Her Arms chatroom so as to describe how amazing Jamie Tworkowski is. He's not just amazing, he's not just an inspiration, he is, essentially, raddtacular.
Jordyn: I need words to describe Jamie of TWLOHA.
Sarah: He's amazing. He's an inspiration. He's raddtacular.