The Latin letter Z, is not a part of the Russian Cyrillic alphabet. But it has been widely noticed on Russian tanks and other armour that have rolled into Ukraine. A nationalistic fervour has taken over in Russia around the letter Z. According to reports, anybody who wants to support Russia in its war efforts in Ukraine are sporting the letter on the shirts, cars etc within the country. Even outside too. (https://www.thenationalnews.com/world/2022/03/07/what-does-the-letter-z-written-in-white-on-russian-vehicles-mean/)

Recently Russian gymnast Ivan Kuliak finished third at the Artistic Gymnastics World Cup in Qatar. The winner was a Ukrainian gymnast. Kuliak, chose to affix what looked like Z on his shirt before standing next to the winner on the podium. That was enough for the organizers to initiate disciplinary proceedings against the Russian, including a long ban. Before this, Svetlana Khorkina, a Russian Olympic legend with seven Olympic medals between 1996 and 2004, shared the "Z" symbol online with the caption: "A campaign for those who are not ashamed to be Russian." (https://www.theguardian.com/sport/2022/mar/07/shocking-behaviour-russian-gymnast-shows-z-symbol-on-podium-next-to-ukrainian-winner)

It remains a mystery how Z came to symbolize support for Russian President Vladimir Putin and the country's military. According to the unverified explanation of the Z phenomenon, Z is interpreted as Za pobedy (for victory) in Russian. Others see it as meaning Zapad (West). Speaking to British television news channel Sky News, Professor Michael Clarke, former director of the defence think tank Rusi, said that often these symbols could be location-based, communicating where a unit was heading. Or it could be as simple as a mark to recognize Russian war machines on the ground. (https://www.express.co.uk/news/world/1576487/Russia-Z-symbol-meaning-evg)

According to news reports from Russia, Pro-Kremlin Telegram channels have incorporated the letter "Z" in their names since the beginning of the invasion, indicating that this has moved beyond being just a mark on military vehicles.

Read more:

Russian tank markings: What do the 'Z' and other emblems on military vehicles mean:
https://www.independent.co.uk/news/world/europe/russian-military-tank-marking-z-b2022276.html

How the letter Z became a symbol for pro-war Russians:
https://fortune.com/2022/03/07/russia-z-tank-marking-how-letter-became-symbol-pro-ukraine-war-invasion/