noun
contract-free mobile phone; unprogrammed cellphone; SIM-free phone
Refers to a mobile phone sold without a carrier contract, SIM lock, or assigned phone number. Often used in the second-hand market to indicate a clean device ready for activation with any carrier.
中古の白ロムを買って、格安SIMを入れた。
I bought a used contract-free phone and put in a budget SIM card.
白ロムなら、好きな通信会社を選べる。
With a SIM-free phone, you can choose any carrier you like.
赤ロム is a phone that has been reported lost or stolen and is blacklisted, making it unusable on networks; 白ロム is the opposite, a clean device.
From 白 (white, implying clean or blank) and ROM (read-only memory), originally referring to a phone with no carrier-specific programming or contract. The exact origin is uncertain but likely emerged in Japanese mobile phone culture.