Englishfor English speakers
informal
Adjective
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If something is informal, it is friendly and casual; it is not official.
Can we get together for some informal talks before the meeting?
School is generally quite informal. Many students even call their teacher by their first name.
theory
Noun
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A theory is an idea that tries to explain an observation.
The new information showed that his theory about growth was incorrect.
Nobody has disproved Einstein's theory of general relativity.