Crosstalk is Konaka's word for the phenomenon of when multiple voices can be heard having conversations at the same time. Crosstalk scenes appear several times in Serial Experiments Lain, the first time being in Layer 03. In the anime, crosstalk is used to convey Lain's forays into the Wired, when she does research or eavesdrops on conversations.
An event that may be a foreshadowing of crosstalk occurs in Layer 01. When Lain is on the train, she hears muffled, jumbled-up voices, even though no one on the train appears to be talking. This suggests that the ability to hear net chatter is a passive ability of Lain's, brought to its full potential when she obtains the Psyche chip in Layer 03. (In the scenario for Layer 01, Konaka compares the voices to those heard by the main character of the 1981 film Scanners before he learns to control his telepathic powers.)
Crosstalk in the anime is usually accompanied by typographics.
In the game, Lain's ability to hear voices coming from phone wires and electromagnetic waves is more explicitly referred to, the first time being in Cou020. When the player idles in the game, it will periodically cut to audio files of crosstalk, accompanied by images of utility poles and power lines.