A theory that states it is possible to make money by buying
securities, whether overvalued or not, and later selling them
at a profit because there will always be someone (a bigger or
greater fool) who is willing to pay the higher price.
Investopedia Says:
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When acting in accordance with the greater fool theory, an
investor buys questionable securities without any regard to
their quality, but with the hope of quickly selling them off
to another investor (the greater fool), who might also be
hoping to flip it quickly. Unfortunately, speculative bubbles
always burst eventually, leading to a rapid depreciation in
share price due to the selloff.