Memory chips that store data long-term. This type of memory is non-volatile, meaning it keeps data even when the device is powered off.
Flash memory in phones is also called flash storage, or simply "storage", and is distinct from RAM. Flash storage is where all of your software and content is stored, whether the phone is on or off. But RAM is generally faster, so that is where a copy of apps and data are stored temporarily while in active use.
The memory inside removable memory cards is also a kind of flash storage.
Flash memory in mobile devices is also sometimes referred to as "ROM", although that term is not technically accurate.