EUR/USD is the single most traded instrument in the currency market, and the nickname arrived with the euro in 1999. Some desks also call it the single currency pair or simply euro-dollar, which is not to be confused with the eurodollar-market for offshore dollar deposits.
Fiber's depth means the tightest spreads and the smallest average daily range in percentage terms among the major-pairs. It is driven by the interest rate gap between the European Central Bank and the Federal Reserve, and by risk-on-risk-off flows.
Example: EUR/USD's average daily range runs roughly 50 to 80 pips in quiet periods, versus 90 to 130 on cable. Position sizes are usually larger on fiber for the same dollar risk.
Related: cable, currency-nickname, major-pairs