[Patashu]

First, there's a letter for each window...

Super Ridiculous has no letter abbreviation and is rarely even seen.
R = Ridiculous
M = Marvelous (Fantastic in ITG)
P = Perfect (Excellent in ITG)
G = Great (or sometimes good)
OK and NG are holding and not holding (No Good) a freeze respectively.

A FC (Full Combo) is going through a song with no CBs (Combo Breakers), those being goods, boos, misses and NGs. (Technically NGs aren't combo breakers but they're an imperfection in your run.) Going further, a full combo can be categorized by the lowest window you hit during this run, leading to Full Great Combos (these are just shortened to FCs), Full Perfect Combos (AAAs), Full Marvelous Combos (AAAAs), Full Ridiculous Combos (AAAAAs). There are also Single-Digit runs, where you're less than 10 of a certain window away from getting one of these AAA+ ranks. SDG for Single Digit Greats SDP for Single Digit Perfects, SDM for Single Digit Marvs. Single Digit CBs also pops up in score discussions.

Finally, there are Attacks, a ratio between that window and the windows below it. PA = Perfect Attack, how often you got a perfect or above. MA = Marvelous Attack, similarly, and RA = Ridiculous Attack.

However, this takes on a completely different meaning for lower windows. Great Attack and Good Attack are aiming to land exactly inside that window on purpose, with the intent on getting a crappy score with all greats or all goods.