It can be either. You can have a monthly salary or an annual
salary.
Depending on where you work you may be paid monthly, or every
two weeks, or twice a month.
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"Monthly" is an adverb or adjective, depending on whether it
modifies a verb or a noun. Examples:: (modifying a noun) You missed
the last monthly meeting. (modifying a verb) Our club meets
monthly.
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The monthly cost of a GoPhone will vary depending on the plan
that a person decides to choose. The average monthly cost can vary
between $20 and $30.00.