A switch is an on-off device in an electric circuit.

Answer

**Step-by-Step Solution:**

1. **Understanding the Function of a Switch:**
- A switch is a device that can either allow or stop the flow of electric current in a circuit. It has two positions: 'on' and 'off'.

2. **Analyzing the Circuit:**
- Consider a simple electric circuit that includes a battery, a bulb, and a switch.
- When the switch is in the 'on' position (closed), it completes the circuit, allowing electric current to flow from the battery to the bulb, causing the bulb to light up.

3. **Effect of Turning the Switch Off:**
- When the switch is turned 'off' (opened), it breaks the circuit. This interruption stops the flow of electric current, and the bulb will not light up.

4. **Conclusion:**
- Since the switch can control the flow of current by being either 'on' (allowing current) or 'off' (stopping current), we can conclude that the statement "A switch is an on-off device in an electric circuit" is true.

**Final Answer:**
The statement is true.

---