some dictionaries assert proper use of language, others just report it. which is why after years of misuse, the word biannual has come to mean *both* twice a year and every other year. the proper definition, the original, is every other year. semiannual means twice a year (literally half year).
all comes straight from the latin. bi meaning two, annus -or- annial meaning year. both annus (annual) and annial (-ennial) mean a year. they mean the same thing.
the term "bi" does it doesn't divide *into* two --it is two. think bicycle (two cycles, or "wheels"), binary (two digits), bilingual, biped, biathlon, bi-provincial, binoculars, bisexual, bifocals (having *two* focal points!), bigamist, bifurcate (literally two fork, to fork or branch into two e.g. becomes two, not divide in half)...
so the bi- prefix on biannual "divides into two"? find me anywhere, where bi means "divide in two". i'll take any example, and not the mere assertion by The Columbia Guide to Standard American English. they punted and got this one wrong.
also, if bi means 'twice', then biannual is "twice year". almost sounds like twice *a* year, but it's not... it's "a twice year". odd way of saying it, but doesn't make the other correct. it's not twice year-ly either. there's no "per" in biannual. it's not "bi-per-annual".
the problem is people have bandied about the terms like biannual incorrectly so much it's losing it's original meaning now. some of the literati acknowledge this and recommend avoiding those terms all together now..
i think some of the confusion stems from the fact it seems natural to rearranged the terms in your head -- take "biweekly meeting" for example, maybe folks are inadvertently associating the "bi" with meeting. it's not bi-meeting-per-week (substituting "per" for "ly") --it's bi-week-per-meeting.
probably even *more* confusing, when you use "meetings" in the plural form (e.g. "biweekly meetings"). it's not because there's more than one meeting in that period, but since all the meetings form a collection, a series. really, i'm only projecting here, when i was confused that's how i started to think about the terms until i thought about it and worked it out.
it *really* gets my goad when people point to biennial saying, "well since biennial means every-two-years, let's have our cake and eat it too and just make biannual mean twice a year, kinda like let's have one term for this, and one for that". grr... truth is, we just don't study latin anymore.
these are the same folks, if you point out the term semiannual already serves the purpose they propose that one evenly divides the year (semi) while the other doesn't. now we're just making up excuses to prop up the wrong definition.
keep in mind that publishers and scientists adhere to the original definition. when you subscribe to a biannual journal, you'll get a copy every other year. biweekly still tends to mean every other week, as bimonthly every other month.
sure language is ephemeral. people frequently use terms incorrectly and these terms take on a new definitions. but when you hear me use the term biannual, i mean every other year.
this is the same problem with the term megabyte. the definition, was definited by the science community, is that mega = 1,000,000. they made the term up. the problem here is programmers (that deal with memory in base 2) -- well they like the fact, or rather the convenience that 2^20 ~ 10^6.
that is...1,048,576 is *nearly* equal to 1,000,000. "within epsilon of delta" anyway, and that's just fine by them.
but it's not cool for the science community, to dilute the meaning of kilo, mega et al… so to fix the problem scientists have put forth new terms for the computer engineers to use :
kibibyte KiB 2^10
mebibyte "MiB" 2^20
etc.
sounds dorky. but most scientists and computer engineers are pretty dorky anyway. nobody i know uses these terms yet, but nobody is counting bytes like days past either.
finally, it's *not* having a change in "tack" -- it's having a "change in tact", or of tactics. here, tack is an intransitive verb used as a nautical term. sailors change tacks to sail into the wind (usually creating a zig zag pattern).
you could say "change of tack",or "taking a new tack", but not a change in tack (implies there's a change within the current tack your taking).