Flickr? Or Flicker?

Flickr isn't an actual word aside from being the name of the photo sharing site. It's a misspelling of flicker, possibly done because the domain flicker.com was already registered at the time. Flickr.com and flicker.com both point to Flickr now, but for the first several years that Flickr was around flicker.com was a completely separate site. Yahoo has owned Flickr since 2005, but they didn't get the flicker.com domain until 2010. By that point, the site was so well established that they probably saw no reason to rebrand and correct the spelling.