Prefix · quantity · equal
Means "equal" or "same": isotope, isosceles, isometric, isobar, isotherm, isomer.
Written by Bryan López, English teacher · Updated
Iso- comes from Greek "isos" (equal, identical) and appears almost exclusively in scientific and technical vocabulary. Isotope = equal position (on the periodic table); isosceles = equal legs (two equal sides); isometric = equal measure; isobar = equal pressure; isotherm = equal temperature. Spanish uses the same prefix: isótopo, isósceles, isométrico, isobara, isoterma, isómero.
"Isometric" is common in fitness (isometric exercises = muscle contraction without joint movement). Weather maps show "isobars" (lines of equal pressure). Chemistry uses "isomer" (same molecular formula, different structure). Knowing iso- unlocks dozens of technical terms at once.
Combines mainly with Greek and Latin roots in scientific contexts:
Pronunciation: /ˈaɪ.soʊ/. Spanish uses the identical prefix: isótopo (isotope), isósceles (isosceles), isométrico (isometric), isobara (isobar), isoterma (isotherm), isómero (isomer). All cognates are direct.
iso-/ˈaɪ.soʊ/
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Classic false etymology trap. "Isolate" derives from Italian "isolato" and Latin "insula" (island). It has no connection to iso- meaning equal. True iso- appears only in scientific terms: isotope, isosceles, isobar, isotherm, isometric.
A trick to remember it
Iso- = equal, same. Spanish uses identical forms: isótopo, isósceles, isométrico, isobara, isoterma. Key rule: in any scientific iso- word, look for the idea of equality or identical value in its meaning.
Exercise 1 · Form the word
Exercise 2 · Pick the right one
How many equal sides does an isosceles triangle have?
The prefix iso- means "equal" or "same": isotope, isosceles, isometric, isobar, isotherm, isomer. In Spanish it usually maps to iso-.
The prefix iso- is pronounced /ˈaɪ.soʊ/. For example, "isotope".
"tope" becomes "isotope". It is a typical example of the iso- prefix.
un-, re-, pre-, dis-, over-, in-... every beginning you need to unlock thousands of English words at once.