High impedance
In electronics, high impedance means that a point in a circuit (a node) allows a relatively small amount of current through, per unit of applied voltage at that point. High impedance circuits are low current, high voltage, whereas low impedance circuits are the opposite.
Digital electronics
In digital circuits, a high impedance (also known as hi-Z, tri-stated, or floating) output is not being driven to any defined logic level by the output circuit. The signal is neither driven to a logical high nor low level; this third condition leads to the description "tri-stated". Such a signal can be seen as an open circuit (or "floating" wire) because connecting it to a low impedance circuit will not affect that circuit; it will instead itself be pulled to the same voltage as the actively driven output. The combined input/output pins found on many ICs are actually tri-state capable outputs which have been internally connected to inputs (resulting in three-state logic or four-valued logic). This is the basis for bus-systems in computers, among many other uses.
The high-impedance state of a given node in a circuit cannot be verified by a voltage measurement alone. A pull-up resistor can be used to try to pull the wire to high and low voltage levels. If the node is not in a high-impedance state, extra current from the resistor will not significantly affect its voltage level.
Analog electronics
In analog circuits a high impedance node is one that does not have any low impedance paths to any other nodes in the frequency range being considered. Since the terms low and high depend on context to some extent, it is possible in principle for some high impedance nodes to be described as low impedance in one context, and high impedance in another; so the node (perhaps a signal source or amplifier input) has relatively low currents for the voltages involved.
High impedance nodes have higher thermal noise voltages and are more prone to capacitive and inductive noise pick up. When testing, they are often difficult to probe as the impedance of a scope or meter can heavily affect the signal or voltage on the node. High impedance signal inputs are characteristic of some transducers (such as crystal pickups); they require a high impedance load from the amplifier to which they are connected; vacuum tube amplifiers, and Field Effect Transistors, more easily supply high-impedance inputs than Bipolar Junction Transistor-based amplifiers (although current buffer circuits or step-down transformers can match a high-impedance input source to a low impedance amplifier).