The thread at http://talk.collegeconfidential.com/community-forum-issues/2021886-common-acronyms-and-abbreviations-used-in-the-college-confidential-forums.html says to post here for any additions or corrections, but the thread is locked.
The article it links to has the usual problem of being unreadable on most browsers other than Chromium on desktop.
Anyway, here are some comments:
AA is also used to refer to (commonly) African American or (less commonly) Asian American.
ACT is a national college admissions examination, formerly American College Testing
AP is Advanced Placement, referring to a high school course covering college frosh level material and following a prescribed syllabus from the College Board, or the exam taken at the end of the school year after completing such a course; colleges may give credit, subject credit, and/or advanced placement for sufficiently high scores on the exam.
CMU can also mean (less commonly) Central Michigan University
CSU can also mean (less commonly) Colorado State University
D1 or D-I can mean (NCAA) Division I; analogous for D2, D-II, D3, D-III.
EC is more commonly used to mean extracurricular
ELC is now defined as having a UC-recalculated GPA higher than the top 9% benchmark set by a recent previous class at the same California high school
FBS and FCS are Football Bowl Subdivision and Football Championship Subdivision of NCAA Division I.
HL is (International Baccalaureate) Higher Level course or exam. Sufficiently high scores on the exam may cause colleges to give credit, subject credit, and/or advanced placement.
IGETC or Intersegmental General Education Transfer Curriculum (not Credit) is the set of course areas that can be fulfilled at a CCC to be counted as satisfying general education requirements if the student transfers to a CSU or UC (although a few UC majors or divisions do not accept IGETC, and some have a small number of additional general education requirements beyond what can be fulfilled by IGETC).
OSU can also mean (less commonly) Oklahoma State University or Oregon State University.
OU is Ohio University or University of Oklahoma (not Oklahoma State University).
Quarter system is an academic calendar where the academic year is divided into three 10-week quarters (instead of two 15-week semesters). The summer term is fourth 10-week quarter, unlike in a semester system where the summer term is typically a shorter term.
REA is Restrictive Early Action, where the applicant agrees to limit other early applications (rules may vary, but commonly no ED anywhere, and often no EA to other private schools) (see also SCEA)
SAT is a national college admissions examination, formerly named Scholastic Aptitude Test, Scholastic Assessment Test, SAT I, SAT Reasoning Test.
SAT II is an obsolete term for SAT Subject Tests.
SAT Subject Tests are tests covering material in high school level courses, formerly named Achievement Tests and SAT II.
SCEA is Single Choice Early Action, where the applicant agrees to limit other early applications (rules may vary, but commonly no ED anywhere, and no EA to other private schools) (see also REA)
SEAS is School of Engineering and Applied Science. Such named divisions exist at Columbia University and University of Pennsylvania
SEC is Southeastern Conference, an NCAA Division I FBS “Power 5” conference.
Semester system is an academic calendar where the academic year is divided into two 15-week semesters. The summer term is typically a shorter term where students who stay take fewer courses or credits than during a regular semester. Some schools may have a very short winter term as well. Semester system is the most common academic calendar in the US.
SL is (International Baccalaureate) Standard Level course or exam.
The Ivy League is an NCAA Division I FCS conference that includes Brown, Columbia, Cornell, Dartmouth, Harvard, Penn, Princeton, and Yale.
UCB is University of California, Berkeley; commonly referred to as Berkeley or Cal.
UT is University of Texas, Austin or University of Texas (system)
UTA is University of Texas, Arlington or (uncommonly, some consider it a mistake) University of Texas, Austin
UTD is University of Texas, Dallas
UW is University of Washington, University of Wisconsin, or University of Wyoming. (some posters do not give enough context to clearly indicate which)