Short answers, pulled from the story.
Sacramental bread is also called Communion bread, Communion wafer, Sacred host, Eucharistic bread, or simply the host. The Latin word hostia, from which "host" derives, means sacrificial victim.
Eastern-rite churches such as Eastern Orthodox and Eastern Lutheran churches use leavened bread, which represents the risen Christ. Western traditions including the Roman Rite of the Catholic Church and most Lutheran churches use unleavened bread, following the model of the Jewish Passover.
Transubstantiation is the Catholic theological teaching that at the Words of Institution, the substance of the bread is changed into the Body of Christ. Eastern Christian theology generally holds that this change occurs at the epiclesis, the invocation of the Holy Spirit.
Yes. In 1995, Cardinal Joseph Ratzinger, then Prefect of the Congregation for the Doctrine of the Faith, wrote to Episcopal Conferences stating that low-gluten bread is valid matter for hosts as long as no additional substances alter the nature of the bread. Low-gluten hosts have been manufactured in the United States, especially in parts of Missouri and New York, since the 2000s.
The prosphora is the leavened sacramental bread used in Eastern Orthodox practice. It is made from fine white wheat flour, pure water, yeast, and salt, and must be baked by a believing Orthodox Christian in good standing, accompanied by prayer and fasting.
The pihta is a small, round, biscuit-sized sacramental flatbread used in Mandaean rituals. It can be salted or saltless depending on whether the ritual is for the living or the dead, and is consumed by the baptized to restore their connections with the World of Light after the Sunday baptismal mass called maṣbuta.