Ehad is the Hebrew word for "one" (masculine?). It is part of several key prayers, including the Shema. Its special significance is due in part to the fact that Judaism was one of the few monotheistic religions at the time of its founding. Ehad is used in prayers like the Shema to affirm the "oneness" of G-d.
The letter aleph is also used as the number 1. An aleph looks like א.