Englishfor English speakers
expression
Noun
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An expression is a group of words, like a phrase or sentence, often one that is used by many people.
"Read between the lines" is a common expression for knowing meanings that are not written or spoken.
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An expression is a group of numbers and variables.
The expression "2x + y - 3" was set equal eight in the equation.
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.