Fantastic was a word used to describe something good or otherwise extraordinary. It was a term used frequently by the Ninth Doctor, as a catchphrase. (TV: Rose, et al.) Unlike the catchphrases of his successors, such as "Allons-y" or "Geronimo", "Fantastic" could be used in a sentence as well as being its own standalone statement.
Jack Harkness used the term to describe the Ninth Doctor to the Sixth Doctor. (AUDIO: Piece of Mind)
The Doctor normally used it during moments of excitement, regardless of the situation at hand. Notably, when faced with a dangerous situation alongside Jabe, he described it as "fantastic", to which she incredulously responded "In what way is that fantastic?" (TV: The End of the World)
When the Eleventh Doctor contacted the Ninth, the latter muttered a sarcastic "fantastic" under his breath, which Rose believed to be the first time he ever used the word in such a way. (AUDIO: Night of the Whisper)
Immediately before his regeneration into his tenth incarnation, the Ninth Doctor used the word to describe Rose Tyler, and then himself. (TV: The Parting of the Ways)
His next incarnation would later use the word to describe the adventures he and Rose would have together, further assuring her he was still the same man after regenerating. (TV: The Christmas Invasion) The Tenth Doctor himself would receive a compliment from his predecessor as having been "absolutely fantastic." (COMIC: The Forgotten)
Jack Harkness described the last remaining humans in the universe as "fantastic" to the Tenth Doctor. (TV: Utopia)
Mr Clever used the term while imitating the Ninth Doctor's accent, during his chess game against the Eleventh Doctor. (TV: Nightmare in Silver)
The Eleventh Doctor would use the term to describe his ninth incarnation to Alice. (COMIC: The Promise)
The Tenth and Twelfth Doctors believed that a Continuity bomb was ineffective in finding a timeline out of billions where the Ninth Doctor's actions weren't "anything less than fantastic." (COMIC: Four Doctors)
When the Twelfth Doctor was forced to degenerate, the briefly emerged Ninth Doctor claimed that the energy required to maintain such a localised time storm was "fantastic". (AUDIO: The Lost Magic)