Universal Integrated Circuit Card (UICC), Embedded Universal Integrated Circuit Card (eUICC), and Integrated Universal Integrated Circuit Card (iUICC) are the software component of the SIM used by modems to attach to a cellular network.
A traditional UICC has limited memory (64 KB or 128 KB), allowing only a single MNO/MVNO profile, while an eUICC has additional memory (512 KB or more) and capabilities that allow it to store multiple MNO/MVNO profiles along with ability to add, remove, and switch between them. Additionally, eUICC has the ability to provision a SIM with a new MNO/MVNO profile over-the-air (OTA).