NTR

In Japanese, NTR is an abbreviation for netorare 寝取られ or netori 寝取り.

Manga: Kobayashi-san Chi no Meidoragon 小林さんちのメイドラゴン (Chapter 6, トールとカンナ)

• shitteru...
  知ってる･･･
  [I] know.
• anaga ga taburakashita
  あなたがたぶらかした
  You seduced [her].
• karada de
  体で
  With [your] body.
• netorare
  ＮＴＲ
  The act of having one's lover taken by someone else. (literally.)
  You're taking her from me. (in this context.)
• nn!?
  んん！？
  Hmmm!?

Manga: Seitokai Yakuindomo 生徒会役員共 (Volume 2)

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.