NTR

The abbreviation NTR stands for netorare 寝取られ or netori 寝取り.

Besides NTR, the abbreviations NTL, NTRR, and NTRS also exist.

NTL

The term NTL means netori. This is because some people believe having NTR abbreviate both netori and netorare is confusing, so for they use NTR for netorare, and NTL for netori.

NTRR

The term NTRR means netorare. This is because some (other) people believe having NTR abbreviate both netori and netorare is confusing, so for they use NTRR for netorare, and NTR for netori.

(what a trainwreck!)

NTRS

The term NTRS is an abbreviation of abbreviation of netorase 寝取らせ, which is a different type of netorare.