Final answer:
The photoelectric effect is a phenomenon that occurs when certain frequencies of light are shone onto a metal surface and cause the ejection of electrons from that metal.
Step-by-step explanation:
The photoelectric effect is a phenomenon that occurs when light shone onto a metal surface causes the ejection of electrons from that metal. It was observed that only certain frequencies of light are able to cause the ejection of electrons. This minimum frequency needed to cause electron ejection is referred to as the threshold frequency.