Light Dependent Resistor (LDR)
From DT Online
The resistance of Light Dependent Resistors (LDR) decreases when light falls on them.
A typical example is the ORP 12 type which will vary its resistance from about 2 Megohms in the dark to 100 ohms in normal room lighting.
LDRs are frequently used as sensors in light activated switch circuits, for example, to switch on lights in the evening when it starts to get dark.