A term coined by Otaku and Weeaboo alike for their 2D significant others; predominantly anime and video game characters. A Waifu, in contrast to a harem, is the love between one man and his one and only Waifu. You treat your Waifu with the utmost respect and courtesy and most refuse to even fap to their beloved, seeing the act as an insult to the non-existent woman they have committed themselves to.
A spin-off term "Husbando" followed for Fujioshi who feel similarly towards male characters.
Not even a missing dimension will keep you from your glorious waifu - The Melbourne MX Newspaper