1) In telecommunications generally, a probe is an action taken or an object used for the purpose of learning something about the state of the network. For example, an empty message can be sent simply to see whether the destination actually exists. Ping is a common utility for sending such a probe.
2) A probe is a program or other device inserted at a key juncture in a network for the purpose of monitoring or collecting data about network activity.
3) Relative to computer security in a network, a probe is an attempt to gain access to a computer and its files through a known or probable weak point in the computer system.
4) In semiconductor testing, a probe card is a microchip placed in a circuit in order to test its signals.