osoroi (お揃い,おそろい)

Osoroi (お揃い, おそろい) is the adjective form of sorou (そろう), a verb whose definitions range from "be complete" to "be equal; be the same."  Osoroi can simply be defined as "matching."

If two or more people are wearing matching shirts, their online photo might be captioned with "osoroi no fuku" (おそろいのふく) or "osoroi no shatsu" (おそろいのシャツ) to denote the matching clothes.

The friend who taught me this word likes to mix Japanese and English; her Facebook post read "We are おそろい today" and was punctuated with a purikura of her and her friend wearing matching outfits.  She also said that girls would be more likely to use the abbreviated form.  In her words, "All Japanese gals want to have おそろ things among friends or with their boyfriend."  Perhaps not surprisingly, search results show more おそろい/おそろ posts by females and couples; it seems less widespread for guys to post pictures of themselves wearing matching outfits and/or accessories. 

If you Google 「おそろいの友達」 (osoroi no tomodachi), the pictures that come up in the Images section will probably include a lot of friends wearing matching shirts, jackets, Mickey/Minnie Mouse head wear, etc.  

Below are some examples, both commerce and blog sites.

http://www.wegoblog.jp/webstore/122591
http://tdrhack.com/hacks/girls-disney/
http://ameblo.jp/earl-juntarou/entry-11654583180.html
http://ameblo.jp/hitomi-endo0614/image-11718193212-12771004742.html
http://aizawaemiri.com/?p=13831

And this clip is from Waratte Iitomo! (笑っていいとも!).  You'll see the matching おそろいuniforms.  The model Rora (ロラ) who appears in this is insanely popular with my teenage and 20-something students, especially the female ones.

http://www.youtube.com/watch?v=EIJ-E4PMdc4