This is by no means a comprehensive list, but these are the most common acronyms you’ll come across in the TEFL world, and I’ve included a few especially pertinent to the CELTA course. If you know of others that ought to be included, do let me know. Thanks!

ALM Audio-Lingual Method
ALTE Association of Language Testers in Europe (Cambridge)
CAE Certificate in Advanced English (Cambridge)
CALL Computer Assisted Language Learning
CCQ Concept checking question
CEFR Common European Framework of Reference
CELTA Certificate in Teaching English to Speakers of Other Languages, formerly Certificate in English Language Teaching to Adults
CLIL Content and Language Integrated Learning
CLT Communicative Language Teaching
CPD Continuing/Continuous Professional Development
CPE Certificate of Proficiency in English (Cambridge)
DELTA Diploma in English Language Teaching to Adults
DM Direct Method
DOS Director of Studies
EAL English as an Additional Language
EAP English for Academic Purposes
EFL English as a Foreign Language
EIL English as an International Language
ELF English as a Lingua Franca
ELL English Language Learning
ELT English Language Teaching
ESL English as a Second Language
ESOL English for Speakers of Other Languages
ESP English for Specific Purposes
FCE First Certificate in English (Cambridge)
GLP Used in CELTA to mean Guided Lesson Planning
GTM Grammar Translation Method
GW Group work
IATEFL International Association of Teachers of English as a Foreign Language
ICQ Instruction checking question
ICT/IT Information [Communication] Technology
IDLTM International Diploma in Language Teaching Management
IELTS In-service Certificate in English Language Teaching. Also: International English Language Testing System
IPA International Phonetic Alphabet
IWB/WB [Interactive] Whiteboard
KET Key English Test (Cambridge)
L1 Student’s first language
L2 Student’s second language
MPFA Meaning, Pronunciation, Form, Appropriacy (referring to teaching language)
OHP Overhead Projector
PACS Post-activity Correction Stage
PET Preliminary English Test (Cambridge)
PGCE Post Graduate Certificate in Education (UK)
PLN Personal/Professional/Passionate Learning Network
PPP Presentation-Practice-Production method
PW Pair work
SLT Situational Language Teaching method
STT Student talking time
SW The Silent Way
TBL Task-based Learning method
TBLT Task-based Language Teaching
TD Teacher Development
TEFL Teaching English as a Foreign Language
TESL Teaching English as a Second Language
TESOL Teaching English to Speakers of Other Languages. Also: Teachers of English to…
TKT Teaching Knowledge Test
TOEFL Test of English as a Foreign Language
TOEIC Test of English for International Communications
TP Used in CELTA to mean Teaching Practice
TPR Total Physical Response
TTT Teacher talking time. Also Test-Teach-Test, a variation of the PPP method.
YLE Young Learners English (Cambridge has 3 YLE exams: starters, movers & flyers)