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.
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)
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)
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)