The 45 is a nickname given to President Donald Trump by his critics as a way to refer to him and the administration without invoking his actual name. The number 45 signifies the ordinal number of Donald Trump's presidency in counting order since the establishment of the office in 1789.

The association between the number 45 and Donald Trump was established shortly after his victory in the presidential election in November 2016. One of the earliest notable instances can be attributed to Trump himself, who was seen sporting a custom edition of his iconic Make America Great Again red baseball cap with "45" embroidered on the side while leaving his Florida resort on November 27th, 2016.

However, the widespread usage of "45" as a pejorative nickname for President Trump was first introduced by American actor Laurence Fishburne during his guest appearance on Comedy Central's The Daily Show With Trevor Noah on January 26th, 2017.

Fishburne: We… we don't need to spend any more time worrying about what 45 is doing, or what 45 and his administration is doing--

Noah: Wait, that's hilarious, you don't call him by his name?

Fishburne: 45.

The day after the airing of The Daily Show. Urban Dictionary HipWaldorf submitted an entry defining "45" as a pejorative alias used to refer to Donald Trump.

On February 24th, 2017, Redditor EverySingleImage submitted a discussion thread to /r/EnoughTrumpSpam, urging people to stop calling Trump the "45" and arguing that using the shorthand more or less legitimizes his presidency. In the following months, a number of signs bearing the number 45 began appearing at major protests and rallies against the Trump administration.