"Seed leaves".  Cotyledons are the modified leaves that develop inside the seed as part of the plant embryo. These are the first leaves to emerge when a seed germinates.  Cotyledons often look very different from the true leaves that develop in the days following germination. 

Alternate vs. Opposite Leaf Arrangement:

When only one leaf is attached to a stem any given spot, it is referred to as alternate leaf arrangement.

When leaves are arranged in pairs, with leaves directly across from each other at a single location on a stem, it is referred to as opposite leaf arrangement.