The X Factor is a British reality television music competition, and part of the global X Factor franchise created by Simon Cowell. Premiering on 4 September 2004, it was produced by Fremantle's British entertainment company, Thames (Talkback Thames until 2011), and Cowell's production company Syco Entertainment for ITV, as well as simulcast on Virgin Media One in Ireland. The programme ran for around 445 episodes across fifteen series, each one primarily broadcast late in the year, until its final episode in December 2018. The majority of episodes were presented by Dermot O'Leary, with some exceptions: the first three series were hosted by Kate Thornton, while Caroline Flack and Olly Murs hosted the show for the twelfth series.

Each year of the competition saw contestants of all ages and backgrounds auditioning for a place, in hopes of proving that they had singing talent. Auditionees attempted to do so before a panel of judges, each selected for their background in the music industry – these have included Cowell, Louis Walsh, Sharon Osbourne, Dannii Minogue, Cheryl, Gary Barlow, Tulisa, Kelly Rowland, Nicole Scherzinger, Mel B, Rita Ora, and Robbie Williams. Those acts who survived the auditions entered a bootcamp stage in which the judges each took charge of a category of contestants to mentor, determining who may move on to the live stages of the contest, with a public vote in the live rounds eliminating these contestants one by one. The winner of the live final received a recording contract with record label Syco Music and a cash payment, though the majority was allocated to marketing and recording costs.

At the same time of its premiere, The X Factor was accompanied by spin-off behind-the-scenes show called The Xtra Factor on ITV2, which focused on the recent episode's performances; this was replaced in 2016 with an online spin-off show, Xtra Bites, on ITV Hub. The programme itself proved popular on British television, attracting high viewing figures at its peak – over 14 million on average in the seventh series – leading to the formation of an international franchise. In addition, many of its acts, including JLS, Little Mix, One Direction and Ella Henderson, went on to release singles that entered number-one in the UK charts.

From 2011, viewing figures began to decline, and Cowell opted to rest the programme in 2019, assigning two spin-offs as mini-series that year – The X Factor: Celebrity and The X Factor: The Band. On 28 July 2021, ITV announced that there were no plans to air another series of the programme, effectively meaning it was cancelled.