Both Uses

convention
in
Hiroshima, by John Hersey
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These thoughts led her to an opinion that was unconventional for a hibakusha: that too much attention was paid to the power of the A-bomb, and not enough to the evil of war.†
Chpt 5

unconventional = not regarded as normal

standard prefix: The prefix "un-" in unconventional means not and reverses the meaning of conventional. This is the same pattern you see in words like unhappy, unknown, and unlucky.
She had led a wildly unconventional life for a Japanese woman of her time.†
Chpt 5 *

Definitions:

(1) (convention as in: teacher's convention)
a large conference or meeting
(2) (convention as in: conventional behavior)
something regarded as normal or typical
(3) (convention as in: The Geneva Convention)
a written international agreement
(4) (meaning too rare to warrant focus)