This article contains plot details about an upcoming episode.
Intrusion Counter Electronics (or ICE in Gibson's stories) refers to the encryption and defence programs of a computer or computer network, similar to the firewalls, anti virus and anti hacking programs of today. They are the programs that protect sensitive data from hackers and any other form of unauthorised access.
In Count Zero the Finn tells Bobby and Lucas that the codes the ENIGMA machine was built to crack were a kind of ICE, saying "so there was ICE before there were computers, you want to look at it that way." Therefore, the term ICE can also mean any method of encryption, computerised or not, that protects data and which has to be decoded for that data to be meaningful and understood. In this sense any kind of coded message can be said to be protected by ICE.
In Gibson's books ICE has evolved beyond encryption and firewalls and can not only prevent intrusion by would be hackers but take action against them as well, even killing intruders if they have been programmed that way. Large corporations and the military have ICE that can track, trace and take hostile action against intruders. The most powerful ICE programs can be seen to have a measure of artificial intelligence, although a "full" AI is supposedly illegal.
In Count Zero, Bobby Newmark is running an Icebreaker program, a program designed to get past the ICE to the data it is protecting, and encounters an especially fierce kind of ICE that almost kills him before he is rescued by Angie Mitchell. Later on in Count Zero, Bobby is told by Jaylene Slide, a top hacker, that Conroy is in a building whose computers are protected by "ICE round them a mile deep", implying that even she cannot hack into it.
In Neuromancer, Case hacks into the computer systems of the wealthy Tessier-Ashpool family with an Icebreaker program from China which does not break through the ICE but slowly takes it over, blending in with it, until it has overcome it, which then allows access to the data the ICE was protecting. It turns out later that the Icebreaker was designed by an AI in order to release itself from the system where it was trapped.