The correct spelling of the synonym to “absolutely” that begins with “Def” is “Definitely.”
“Definitely” means certainly, unquestionably, or without doubt. It comes from “definite,” meaning clearly fixed or settled, with the adverb ending “-ly.” The word ultimately traces back to the Latin definire, meaning “to limit” or “define.”
The most common mistake is writing “definately,” replacing the middle “i” with “a.” A useful memory aid is to spot the word “finite” inside d-e-f-i-n-i-t-e-l-y: if it contains “finite,” it must contain an “i,” not an “a.” “Defiantly” is a real word, but it means rebelliously or in a defiant manner, so it is not a spelling variant of “definitely.” “Defantily” is not a standard English word.
Although claims about the single most-misspelled English word vary by source and writing population, “definitely” is consistently listed among commonly misspelled words. In careful writing, remember: definite becomes definitely, with no “a.”