UNO R3 + WiFi ATmega328P+ESP8266 (32Mb memory) USB-TTL CH340G For Arduino Uno - External ESP8266 Pins
From esp8266 datasheet:

TOUT - ADC pin. It can be used to test the power-supply voltage of VDD3P3 (Pin3 and Pin4) and the input power voltage of TOUT (Pin 6). However, these two functions cannot be used simultaneously.

from the pinout PDF of the Uno+WiFi board: