For those who are not expert in such things, the sandek holds the baby while the mohel circumcises him. I briefly spoke after the bris and explained what I had learned courtesy of Michelle Klein in A Time to Be Born: Customs and Folklore of Jewish Birth. Most of this is in one of her footnotes:
Sometimes the father performs the job of sandek, but more often it is a grandparent or a rabbi or some other person who will serve as a role model for the boy.
The word sandek derives from the Greek syndikos, which is the same word in Late Latin and means a delegate, someone entrusted with a special affair.
This name, which was not mentioned in the Talmud, implies that this role was formalized in Byzantine times. Yalkut Shimoni (Jerusalem: 1960), vol. 2, Psalm. 35 discussed the origins of this word.
Judeo-Arabic speakers have said the word comes from the Arabic word sandouk, meaning a box, because it was once the custom for the mohel to bring all his equipment in a long narrow box.
After emptying the box in the room of the circumcision, he placed it below the mezuzzah, and the sandek sat on this during the rite.
The sandek was thus he who sat on the sandouk.
Another explanation is that sandek is derived from the Hebrew shen dak, literally a "thin tooth," which refers to the thin tooth of flint used by Zipporah to circumcise her son (Exodus 4:25): Ben R. Simhon, 1994, op. cit., p. 92. ...
A kabbalistic interpretation proposed that the four Hebrew letters of the word form the initials of four Hebrew words--sod, nah?ash, dam, kodesh, which depict mystical aspects of the rite, in kabbalistic imagery: Pritzker, A., "Brito shel Avraham avinu," Yeda Am 1 (1954): 22. [repost from 5/06]