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A Tkon energy field

An energy field was a general term used to refer to any defined spatial area that had some sort of energy-related effect within that area. Many energy fields occur naturally in space, but artificial fields could exist as well. Usually technological devices such as field generators, like the one used by the Whale Probe, created the latter, though not in every case; certain (often powerful) lifeforms, such as Q, Armus, and the Sky Spirits, had the ability to generate energy fields at will. (Star Trek IV: The Voyage Home; TNG: "Encounter at Farpoint", "Skin of Evil"; VOY: "Tattoo")

In space

Regions of space can be filled with energy fields. In exploring space, Starfleet vessels often encountered energy fields of one type or another. Some of these fields were easily measurable and quantifiable; others were the product of advanced science or previously unknown phenomena, and exhibited unusual or previously unknown properties.

When the USS Enterprise arrived at the Shore Leave Planet in 2267, scouts from the vessel didn't detect any energy fields on the planet whatsoever. However, the starship then became the victim of an unknown energy field generated by the planet's underground machinery. This unusual field drained energy from the ship, making communication and transportation difficult. It drained most hand-held devices (such as phasers) completely, rendering them useless. As Spock later reasoned, the power field was being used to manufacture multicellular castings which the planet had in abundance. Just after the Keeper discovered that the Enterprise crewmembers had been discomforted and perplexed by the preceding events rather than amused, he apologetically explained the situation to the visitors, and the Enterprise's power banks came back online. (TOS: "Shore Leave")

The energy field presumably also powered antennae which, in the episode, repeatedly appear.

In 2270, the Enterprise encountered and entered an unidentified energy field near the Romulan Neutral Zone to escape three pursuing D7-class Romulan warships. Spock's analysis of the field found that it was composed of highly-charged subatomic particles. It was later determined that these subatomic particles invaded the ship's computer circuits like bacteria invade biological matter, creating an electronic equivalent of a nervous breakdown. (TAS: "The Practical Joker")

The script described the phenomena as a "large, glowing mass like a luminous fog bank".

The USS Enterprise-D encountered an energy field created by the Paxans which was designed to transport intruders away from their space. (TNG: "Clues")

Types of energy fields