Destiny

Destiny was a sometimes religious concept about what will and must happen that not all agreed with. Peri Brown thought that destiny was "just a fancy name for blind chance", while Yrcanos said that, if that were true, life would be meaningless. (TV: Mindwarp)

Destiny was something that must not be messed with. Amy Pond asked her younger self, "You're asking me to defy destiny, causality, the nexus of time itself, for a boy?" The Eleventh Doctor thought it impossible to "defeat pre-destiny", which he did manage to do. (TV: The Girl Who Waited)

When Donna Noble parked her car right next to where the Tenth Doctor would park his TARDIS, she said, "This is like destiny!" (TV: Partners in Crime) The Meta-Crisis Tenth Doctor later confirmed this:

"It's like we were always heading for this. You came to the TARDIS. And you found me again. Your granddad. Your car. Donna, your car. You parked your car right where the TARDIS was going land. That's not coincidence at all! We've been blind. Something's been drawing us together for such a long time."

- Meta-Crisis Tenth Doctor

"Destiny", or rather Dalek Caan's manipulation of time, lead the Doctor to meet Donna twice, and her grandfather, so that Donna could become the Doctor-Donna and the Meta-Crisis Doctor could be created. This became instrumental in the defeat of Davros's plan to destroy all universes through the reality bomb. (TV: Journey's End)

After the Eleventh Doctor lost Clara Oswald again in Victorian London on Christmas 1892, he began searching for another version of her, hoping that, if he just wandered around enough, he'd find her eventually, "like destiny, sort of". A young Clara Oswald, who the Doctor did not recognise, called destiny "rubbish". (WC: The Bells of Saint John: A Prequel)