peripheral
The BLE device that advertises and waits to be connected to, usually the sensor.
The peripheral advertises and accepts connections, and usually acts as the GATT server that exposes its data.
Examples
The greenhouse sensor advertises "I'm the greenhouse sensor" and waits for a phone to connect: it is the peripheral.
A heart-rate chest strap is a peripheral when an ESP32 connects to it to read data.
See also