It stands for English Language Teaching.
Example
Many large editorial companies have ELT sections which publish books for English teachers and learners to use.
Further links:
https://www.teachingenglish.org.uk/article/milestones-elt-a-series-classic-elt-texts-british-council
https://www.teachingenglish.org.uk/article/internet-elt-impact-internet-english-language-teaching