What is the technical term for the delay before an IP camera image appears on screen?

The word you are looking for is Latency.

Latency is a measure of time delay experienced in a system, the precise definition of which depends on the system and the time being measured. Latencies may have different meaning in different contexts.
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Satellite transmission
This is best illustrated when a newsreader in a studio talks to a reporter half way around the world. The signal travels from the newsreader via communication satellite situated in geosynchronous orbit to the reporter and then goes all the way back to geosynchronous orbit and then to the studio, resulting in a journey of over one hundred thousand kilometers . This time lag is easily noticeable. Even though the signal travels at the speed of light, it still requires about half a second to travel that distance (not including the much smaller latencies inside the communications equipment).

Lag is also an acceptable substitute, though it is considerably more informal.

Lag is a common word meaning to fail to keep up or to fall behind. In real-time applications, the term is used when the application fails to respond in a timely fashion to inputs.
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The term lag is often also used as a synonym for communication latency. This can be misleading because there can be other causes for the symptom.

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It's worth noting that the video latency of IP cameras is not entirely due to the camera itself. Depending on the intervening network, network latency can actually produce the majority of the delay in the signal.