An eUICC is an embedded SIM Card which has the eligibility to download multi profiles on it via loading in the factory or downloading them over the air by a Subscription Management Platform/Remote SIM Provisioning Server.
It has anti-tamper security on it because of the hardware that it still uses.
eUICC interoperable profile standards are defined by SIM Alliance in coordination with GSM Association and leading SIM Card vendors design and develop their own eUICCs accordingly.
An eUICC is same as the SIM Card from appearance but the operating system of the eUICCs are quite different than the traditional SIM Card due to the security mechanisms introduced and the ability to download many profiles on one SIM card at the same time.
The eUICC form factors could be as follows;
Plug-in SIM (2FF) 25x15 mm
Micro SIM (3FF) 15x12 mm
Nano SIM (4FF) 8,8x12 mm
M2M/eUICC (MFF2) 6x5 mm