Formal:
"centi-bit" (cBTC) / "milli-bit" (mBTC) / "micro-bit" (µBTC) / "satoshi" (sBTC)
Slang:
"ceebits" (cBTC) / "embits" (mBTC) / "youbits" (µBTC) / "satoshi" (sBTC)
But I think the cBTC one is unnecessary. Also, it'd be easier for µBTC to be written as uBTC.
All these denominations could collectively be called "bits". "How many bits would that be? 20 youbits. Here, thanks."
µ is the official (SI) letter assigned to micro (1*10^-6) just like n is for nano (would be a 1/10th satoshi) and m is for milli, k is for kilo, M (capital for mega, etc......)
u is sometimes used but µ is better.