"Sandek" is a title conferred upon an adult participant in a baby's bris; a title of highest honor. The Sandek holds the baby during the circumcision and/or a significant, specific portion of the greater ceremony. Traditionally, the circumcision was performed while the baby was on the Sandek's lap, but these days it mostly happens on a table.
Some people translate the word to mean "godfather," others prefer "patron." Either is fine. Traditionally the honor was reserved for the baby's grandfather, or great-grandfather, if available. Next choice was the rabbi. Whoever it was, it was always a man, not necessarily a family member, but always an observant Jew from a devout family.
The piety of the Sandek is crucial, because his role is to protect the child from negative forces and help preserve positive energy. Part of the goal of a bris is to liberate a baby from negative spiritual influence and to unite him with his soul, thus giving him a clean start, and the Sandek is the conduit for this. If he is a righteous man, he can help draw down a holy soul for the child. He shares a spiritual connection with the baby, and transfers to him his piety. During the ceremony, he places a corner of his own prayer shawl over the forehead of the baby.
Modern Sandeks are often female, often non-Jewish, often plural. Some choose to honor the baby's grandparents, or both grandmothers.
The word derives from the Greek "suntekos," which translates as "companion of child." "Sandekim" is the plural form.
http://www.grossweb.com/bris.htm
http://www.beritmila.com/sandek.html
http://www.act-now.org/circumcision3.htm
