millibitcoin.  It's metric, like millimeter, milliliter, milligram, etc.

1 millibitcoin (1 mBTC) = 0.001 bitcoin
1 millimeter (1 mm) = 0.001 meter
1 milliliter (1 ml) = 0.001 liter
1 milligram (1 mg) = 0.001 gram

(see the pattern?)


Metric again.  In this case it is microbitcoin.  (like micrometer, microliter, microgram, etc.)

1 microbitcoin (1 µBTC) = 0.000001 bitcoin
1 micrometer (1 µm) = 0.000001 meter
1 microliter (1 µl) = 0.000001 liter
1 microgram (1 µg) = 0.000001 gram

Notice, many people have a difficult time figuriing out how to type a µ symbol with their keyboard, so they use the letter u instead (which looks similar).