extending throughout an entire nation
"nationally advertised"
"it was broadcast nationwide"
In a way relating to the whole nation.
"The new policy was implemented nationally, affecting every state and territory of the country."
In plain English: Nationally means something happens across an entire country instead of just in one place.
"The holiday is celebrated nationally with parades and fireworks across all major cities."
The word nationally comes from combining the adjective national with the suffix -ly. It entered English as a straightforward formation to create an adverb meaning "in a national manner."