Memory that stores data without electrical current, It is the ROM (Read Only Memory) or is sometimes called non-volatile memory as it is not erased when the power is switched off. Data has been prerecorded and once data has been written onto a ROM chip, it cannot be removed and can only be read. ROM is mostly used to store the booting programs. The OS is mostly stored in ROM. ROM contains the programming that allows your phone to be "booted up" or regenerated each time you turn it on.
Like RAM in mobile it cannot be removed from the phone but it can be upgradable. Normally the OS is stored in ROM so Mobiles OS can be upgraded.