What is Energy in 5e?

Question

There are several spells and effects, like Resilient Sphere, that care about energy. In general, when the game uses terms that are not defined explicitly, it defaults to the plain English usage of the word. This would give us a definition such as:

• a dynamic quality; the capacity of acting or being active; a usually positive spiritual force
• vigorous exertion of power
• a fundamental entity of nature that is transferred between parts of a system in the production of physical change within the system and usually regarded as the capacity for doing work
• usable power (such as heat or electricity)

Excluding the psychological energy describing character behaviours, this covers any kind of heat, light, sound, motion. It would include the energy imparted from physical strikes. When looked at closely, this quickly leads to discussions about physics, something the D&D rules are typically not well set up to deal with.

Is this what 5e means when it talks about energy?

Answer 1

"Energy" is not a well-defined game term with a consistent meaning.

As you lay out in your excellently-researched answer, the word means different things in different places. Frequently it refers to damage types including fire, cold, lightning, and thunder. Sometimes it doesn't.

We should not attempt to converge on a consistent definition for this term, because the authors of the books are not using a consistent definition.

Answer 2

Energy in the game are powerful, often magic-fueled effects (and the exact definition is up to the DM)

I only found a single use of the word in the PHB that can be interpreted in the physics sense, on p. 119, under School of Transmutation: "You are a student of spells that modify energy and matter."

In early editions of the game, the term "energy" explicitly referred to magical energy¹ powering spells, from the 1e DMG, p. 81: "Magical power is energy from another plane channeled through this one by the use of certain prescribed formulae." So is this meaning carried on?

There clearly is no conclusive answer about this, as there is no formal definition, but I think there is enough support for a non-physics reading that makes this ruling territory:

In 5e there are many passages that mention energy. The game uses the term with other words, in combinations such as magical energy, positive energy, life energy, divine energy, and clearly refers to something outside the remit of physics. The text is not talking about the common English (i.e. physics) meaning of energy, but about special magical forces.

The Weave of Magic sidebar on p. 202 PHB describes this:

The worlds within the D&D multiverse are magical places. All existence is suffused with magical power, and potential energy lies untapped in every rock, stream, and living creature, and even in the air itself. Raw magic is the stuff of creation, the mute and mindless will of existence, permeating every bit of matter and present in every manifestation of energy throughout the multiverse.

There are many other examples: the divine energy that clerics and paladins draw upon is used to "fuel magical effects" (p. 58 PHB), the Life Domain talks about "vibrant positive energy — one of the fundamental forces of the universe, and the Areas of Effect section in the PHB (p. 201) declares: "Every area of effect has a point of origin, a location from which the spell’s energy erupts.". The dragonborn's breath weapon is described as (p. 34 PHB): "You can use your action to exhale destructive energy", the damage types of that energy being listed as acid, lightning, fire, poison and cold. Poison in particular is a bad match for a physics-leaning interpretation of the term.

In many spells the term energy is used when referring to non-physical damage types, for example:

• Protection from energy, grants resistance to one damage type of your choice: acid, cold, fire, lightning, or thunder, and Hallow's Energy Protection states "Affected creatures in the area have resistance to one damage type of your choice, except for bludgeoning, piercing, or slashing."

• Dragon's breath allows the target to "spew magical energy from its mouth, provided it has one. Choose acid, cold, fire, lightning, or poison."

• Chromatic Orb states you hurl a 4-inch-diameter sphere of energy at a creature that you can see within range. You choose acid, cold, fire, lightning, poison, or thunder for the type of orb

• Glyph of Warding's Explosive Runes says "the glyph erupts with magical energy in a 20-foot-radius sphere (...). A creature takes 5d8 acid, cold, fire, lightning, or thunder damage"

• Various spells such as Arms of Hadar, Blight, Circle of Death, all talk about energy causing necrotic damage.

• Any kind of force damage is explicitly energy (p. 196 PHB): Force is pure magical energy focused into a damaging form.

While the exact types of "energy" differ from spell to spell or ability to ability, they all share that they are strong enough to cause damage. It is radiant energy, causing radiant damage, not just light. It is thunder energy causing thunder damage, not just sound.

Other spells (like auras) have magical or divine energy that heals or protects.

I therefore think that a case can be made that when the game mentions energy, it means something beyond normal sound or light, something caused by magical or divine abilities or something strong enough to cause effects such as damage, healing, protection or transformation.

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¹ The term was also separately used for the life intensity of a creature (levels).