Pillow friends are novices that form an extremely close bond, with a sexual component, though they can also be Accepted. Merean believed that Siuan and Moiraine were pillow friends (New Spring). Most leave such relationships behind them once they are raised (KoD, Ch. 25), but some still maintain an interest in other women. Meidani and Elaida had been pillow friends as novices, and Elaida considered being open to Meidani renewing the relationship, something Tarna disapproved of, though she thought to herself that Galina had been quite surprised when she refused her advances (KoD, Ch. 25). Yukiri considered the possibility that Seaine and Pevara were pillow friends to be their own business (TPoD, Ch. 28). Most people do not particularly disapprove and Cadsuane was amazed to learn Ailil and Shalon had put up with days of questioning to hide their relationship (WH, Ch. 13). Though it is not disapproved of, it is not encouraged, as the relationship can have very severe strains to it (KoD, Ch. 24). Delana and Halima were believed by many to be pillow friends, which Romanda considered a mistake, as she thought it unwise for a sister to take a woman who could not channel as a pillow friend, especially if the other took the lead (KoD, Ch. 23)
Pillow friends are not just good friends. Oh, they are that, too, but they also get hot and sweaty together and muss up the sheets something fierce. By the way, pillow friends is a term used in the White Tower. The same relationship between men or women elsewhere would be called something else, depending on the country (Robert Jordan's Blog, September 30 2005).
"Pillow friends were common among novices and Accepted, but girlhood things should be left behind with girlhood" (Tarna, Knife of Dreams, Chapter 25).