The Latin abbreviation nb (nota bene, "note well") could be used for this purpose.
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Nota bene: you must submit your completed form before
midnight.
Nota bene is frequently abbreviated "NB", so it would also be
fine to write,
"NB: you must submit your completed form before midnight.
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Nota Bene
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NB as in Nota Bene
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The term "nota bene" translates to English as "note well." An
example of a sentence using this phrase would be: We will be in the
Adirondacks this weekend; nota bene it gets very chilly at night in
the mountains.