This is the new Celtic Teamgeist strip 2021, a special edition jersey made for Scottish giants Celtic by kit partners Adidas to mark the 15th anniversary of their Teamgeist design, a template that was used by their teams during the 2006 World Cup and the 2006/07 European club season.
Celtic, interestingly, were outfitted by Nike when the original Teamgeist design came out. So were Arsenal incidentally, but Adidas have used the famous redcurrant colour from their kit of that era for the Teamgeist jersey.
Real Madrid actually used the Teamgeist template back in the 2006/07 season, so their Teamgeist retro shirt is a virtual replica.
Celtic are one of three British teams to get this special retro shirt- Arsenal and Man Utd being the others. A total of nine teams, including continental heavyweights like Juventus, Real Madrid and Bayern have been given Teamgeist releases.