Does the Eastern Orthodox Church eat leavened or unleavened bread during the Eucharist? Does the Eastern Orthodox Church eat leavened or unleavened bread during the Eucharist, and if it is indeed leavened bread, what is their basis for this practice?
Yes. The Eastern Orthodox Church uses leavened bread.


  Leavened bread has always been used in the Eastern Church.
  In fact, at one point in time, a great controversy raged over the fact
  that in the Eastern Church leavened bread was used, while in the
  Christian West unleavened bread was the norm. (Orthodox Church in America)
  
  The Orthodox Church uses leavened bread for, according to the Gospel
  of Saint John, Last Supper and Passion, took place during the evening,
  night and day time of Passover Day, therefore leavened bread was eaten
  in Last Supper. (Orthodox Wiki)


The words used to describe the bread of the Last Supper (and Christ's commandment to do as he had done) are the basis for this.


  If we look at the word for the bread used at the Last Supper is
  artos, the Greek generic term for bread that almost always...though
  not necessarily always referred to every day leavened break. The Greek
  term that specifically means unleavened bread is azymes. This gap is
  cleared up if we look at the earliest Aramaic translations of the
  Gospels (third century or earlier), the word they use to translate
  bread at the Last Supper is not generic, but specifically means
  leavened bread. (Orthodox Christianity, emphases mine)