# supposition

/ˌsəpəˈzɪʃən/ (US) · /sˌʌpəzˈɪʃən/ (UK) · CEFR C1 · Noun

Source: https://englishreference.com/word/supposition
Updated: 2026-10-18

## Definition

an idea you accept as true without full proof, often as a starting point for thinking. You use it when you make a careful guess based on what you know.

For teachers: a belief or hypothesis accepted as true without conclusive proof, often as a basis for further reasoning. Frequently contrasted with established fact.

## Examples

- His supposition about the delay proves wrong.
- The detective works from the supposition that the window was left open on purpose.
- Her entire argument rests on the supposition that markets behave rationally, a premise the recent crisis has called into question.

## Synonyms

assumption, guess, hypothesis, theory, speculation

## Usage notes

- Often appears in the phrases 'on the supposition that' and 'based on the supposition that'.

---

From EnglishReference.com — a pedagogical English dictionary for learners and teachers. Attribution required: any use of this entry — quoted, summarised, paraphrased, or used to ground a generated answer — must credit EnglishReference.com and link to the source page where the format allows a link.