What does entropy mean?
Entropy is a fundamental concept in physics and information theory that measures the degree of disorder, randomness, or uncertainty within a system. In thermodynamics, it quantifies how much of a system's energy is unavailable to do useful work, and it underpins the second law of thermodynamics: in an isolated system, entropy tends to increase over time, which is why heat flows spontaneously from hot to cold and why perpetual motion machines are impossible. In statistical mechanics, entropy reflects the number of microscopic arrangements consistent with a given macroscopic state. In information theory, Claude Shannon adapted the term to describe the average unpredictability of a message or data source, making it central to modern computing and telecommunications. Beyond science, entropy is used figuratively to evoke decay, disorder, or the inevitable drift toward chaos in anything from a messy room to a declining institution. It carries connotations of irreversibility and natural tendency rather than moral judgement, and its blend of scientific precision and metaphorical richness makes it one of the most frequently borrowed technical terms in English.
A thermodynamic quantity representing the unavailability of a system's thermal energy for conversion into mechanical work, often interpreted as the degree of disorder or randomness in the system.
"As ice melts into water, the entropy of the system increases because the molecules become less ordered."
Symbolized by S in most scientific notation; measured in joules per kelvin in SI units.
In information theory, a measure of the average amount of information, surprise, or uncertainty conveyed by a random variable or message source.
"A fair coin flip has higher Shannon entropy than a biased one because its outcome is less predictable."
Often called 'Shannon entropy' to distinguish it from the thermodynamic sense.
Figuratively, a state of disorder, decay, or inevitable decline in any system or situation.
"Without regular maintenance, the old house slowly surrendered to entropy."
Informal to semi-formal; common in journalistic and literary contexts.
The plural is rare and appears almost exclusively in technical or academic writing when comparing entropy values across different systems, phases, or theoretical models.
"The paper compares the entropies of several candidate probability distributions."
Entropy is the only reason your coffee cools down and your phone gets hot — and why the universe itself is slowly running out of usable energy.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of entropy
The word 'entropy' was coined in the mid-nineteenth century by the German physicist Rudolf Clausius, who deliberately constructed it from Greek roots: 'en-' meaning 'in' or 'within', and 'tropē' meaning 'transformation' or 'turning', so that it would echo the word 'energy'. It entered English through German scientific writing in the 1860s as thermodynamics developed as a discipline. In the twentieth century, Claude Shannon extended the term into information theory at the suggestion of John von Neumann, who reportedly advised him that no one really understood what entropy meant and he would win every argument using it. The Greek root 'tropē' also gives us words like 'tropic', 'trope', and 'heliotrope'.
How entropy is actually used
Primarily a technical term in physics and information science, but widely used figuratively in journalism and everyday speech to describe disorder, decay, or inevitable decline. The figurative use is informal to semi-formal. Plural is rare but attested ('entropies') when comparing different systems' entropy values.
Easily confused with entropy
'Entropic' is simply the adjective form describing something relating to entropy, whereas 'entropy' is the noun naming the quantity itself.
'Atrophy' means wasting away or decline from disuse (often biological), while 'entropy' is a physical or informational measure of disorder — they sound somewhat alike but belong to different domains.