A cold opening (or pre-title sequence) was the scene of a story that appeared prior to the title sequence.
With some exceptions, pre-title sequences were not featured in Doctor Who in its original run from 1963 to 1989. The cold opening became a permanent instalment of Doctor Who from the 2005 episode The End of the World onward. Pre-title sequences were completely abandoned during series 11, but were promptly reintroduced in Spyfall: Part One, the opening episode to series 12. Only some of its episodes had it return though.
All episodes of Class began with a cold opening.
Exceptions to the above boundaries
In the classic series, just four stories featured a pre-title sequence. 1982's Castrovalva, the debut story of Peter Davison as the Fifth Doctor, was the first. The twentieth anniversary special, 1983's The Five Doctors was the second. These two stories were followed by 1987's Time and the Rani, the debut of Sylvester McCoy as the Seventh Doctor, and 1988's Remembrance of the Daleks. The 1996 TV Movie and animated reconstructions of 1966's The Power of the Daleks, 1967's The Macra Terror and the incomplete Douglas Adams story Shada also featured cold openings.
In the 2005 version of the programme, the majority of exceptions were series premieres. Rose, Smith and Jones and Partners in Crime which were the series premieres of series 1, 3 and 4 respectively all did not feature cold openings. The fiftieth anniversary story The Day of the Doctor did not feature a cold opening, instead beginning with a slightly modified version of the title sequence from the first Doctor Who television story, 1963's An Unearthly Child.