The thing is that neither frequency nor interval have a specific definition under 13485, 9001, 9000 or even their extended glossary.
Definitions resting outside of these are then not only subject to interpretation, but even to relation with the other terminology.
We most commonly think of frequency being composed of occurrences with equal intervals between them, but it is distribution independent in this sense.
In my mind frequency is merely number of occurrences within a specified range (also look up the frequency function in excel to get an idea).
Interval is the space between two points. Defining an interval in my mind then has more to do with distribution between points (most dictionary entries will point to something similar).
How complex you make the interval in the sense of the standard is also a matter of perspective.
You could take for example any of the following:
a) any QMS part will be audited again within a year it was last audited. (Defining the maximum interval), subject to dividing your QMS up into parts, e.g. clauses, as a prerequisite).
If you decided to 'for-cause' audit it ½ a year after your 'regular' audit, you in essence would reset the counter to have the next audit occur no later then 1½ year after the previous 'regular' audit.
b) Regular audit wills occur yearly, with part A being audited in Q1, part B in Q2, etc.
There's always an audit planned for one year after the last regularly planned audit, no matter if amount of covering audits (and thus frequency)went up due to whatever reason.
c) at least one audit will occur each month.
Letting go of parts, you end up with a simplified obligation. This could be risky for ascertaining coverage over the period, or being abused to have 'light' audits count as heavy as full audits for coverage KPI.
Note that while it does not say
documented planned intervals as with the management review, the audit program paragraph "shall" takes care of this. Note also that the audit program does not only require definition, but now also records (unlike the 2012 edition).
So for your terminology:
"Once a year" would be to me: within a year, at least once. Twice a year would for example still be met if it occurred in october and december, no matter if the previous year would have them at january and march. This is because the year defines a calendar period.
"Annually" would be to me a year (12 months, 365 days) from the last occurrence, as annually matches a length of period, but not a specific starting point. Biannually would be: again, half a year from now. (Biennially would be: again, two years from now).
But then this drifts more into dictionary and linguistics then anything else.
tl;dr:
Marc's interpretation is encountered most often and widely accepted. You don't want to have a vocabulary battle with your auditor, you want their help to become or remain as effective as possible. Hogheaven's farm takes the debate out of any of the above, and is the shortcut as a year is not a requirement from the standard (but it might be a requirement from somewhere else).