Omonia translates to Concorde which is a French word meaning 'agreement' and is named thus to commemorate the amelioration of differences between 2 long forgotten Greek Political parties of the 19th Century.
Omonia is the city of Athens 2nd most important square. Omonia Square or "Concord Square" is somewhat analogous to Times Square in NYC. There is a little bit of everything in the square area including nice hotels, traffic, hustlers and car exhaust. You can buy bootleg tax free cigarettes from the women near the central market and the porno movie theaters are right nearby too on Aghios Konstandinos Street as well as the baths.
I understand they cleaned up Times Square so the analogy may be inexact but you get the general idea. Every big city has an area like this and Omonia is Athens' and one of those places you invariably find yourself at one time or another too. If you know what's available to you its a convenient area and with the metro just below very accessible.