As the Twelfth Doctor described, probability was a branch of mathematics that gauged the likelihood of future events which could not be predicted with absolute certainty. (COMIC: Gangland)
The Time Lords could create a fixed point in time by using their abilities to freeze a probability into a certainty. (PROSE: The Gallifrey Chronicles)
The unusual properties of Tick-Tock World could create multiple equally probable occurrences of a single event. When the First Doctor was devoured by a Xesto, he awoke in a hinterland populated by several alternate versions of himself, which a future version of Susan explained were the victims of equally probable, but slightly different, ways that he could have endured such a fate. (AUDIO: Tick-Tock World)
The computer of a Chula warship estimated a 100% probability that any attempt to jettison a German bomb would precipitate detonation, and that Captain Jack Harkness would be terminated in under two minutes. However, Jack was ultimately retrieved by the Ninth Doctor. (TV: The Doctor Dances)
When in Las Vegas, the Twelfth Doctor explained to Clara Oswald that rigorous probability calculations governed his actions, demonstrating this by using his mathematical skills to win at gambling. (COMIC: Gangland)
Morris Gibbons stated that the fact that Susan Triad's first initial and last name being an anagram of TARDIS, as well as the fact that the shared her first name with that of the Doctor's granddaughter Susan Foreman, meant that the probability of it being a trap was 95% and rising. He later revised this figure to 96%. Later, when UNIT discovered that the mysterious black twister shape seen via a time window had the Doctor's TARDIS at its heart, he gave the probability of a trap as 99% and rising, though noted that the trap was there with them, as the TARDIS was there inside UNIT. (TV: The Legend of Ruby Sunday)