What is contamination? Is it small amounts of dirt in the bulb or oxygen? What'd happen if water gets in? How does it effect the arc?

Contamination happens whenever an undesirable substance is present where it shouldn't. It happens in food, medicines, paint, chemicals, lamps, etc.

Specifically in lamps, it is usually impurities from manufacture that are present in too high a concentration to be benign. Oftentimes, impurities will react with desirable substances in a lamp and render operation less than satisfactory, if not undesirable. According to the pure definition of the word, the presence of air from a leak in the lamp could be considered contamination as well.

Basically anything that isn't mercury argon or in some cases krypton.

Contamination happens whenever an undesirable substance is present where it shouldn't. It happens in food, medicines, paint, chemicals, lamps, etc.

Specifically in lamps, it is usually impurities from manufacture that are present in too high a concentration to be benign. Oftentimes, impurities will react with desirable substances in a lamp and render operation less than satisfactory, if not undesirable. According to the pure definition of the word, the presence of air from a leak in the lamp could be considered contamination as well.

Does fingerprints on the arc tube surface considered contamination? Some caina made mercury lamps have this problem, don't know if it affects lamp life

Definitely it means at least a sloppy quality maindset (not only - because it is apparently tolerated) on their manufacturing floor. With fingerprints you never know what exactly ended up there (who knows what was on the fingers?), so with material known to fail at high temperature after some fingerprint contaminations this is not something I would accept. Plus it also means the workers do not use any PPE in an environment where toxic chemicals are handled (Hg,...). That was never an environment where good reliable emplyees stay for any long, so who remains are only morons and inexperienced staff. So again, what would be the resulting work quality?