bi usually means you have 2 options.

Bi monthly: every other month
Semi monthly: twice a month

Thats cute but you don't know shit about bi monthly. If I had a ukulele I'd write a pithy song about how you don't know what you're talking about.

I’m intrigued. JB is definitionally correct, so let’s play this out.

Oh yeah? Cut and paste for me the definition of bi-weekly. Just for kicks.

Bi weekly: every other week
Semi monthly: twice a month

bi·​month·​ly | \ (ˌ)bī-ˈmən(t)th-lē  \

Definition of bimonthly 

1: once every two months

2: twice a month

I’m fairness, you asked me to define bi-weekly and we’re down a bi-monthly rabbit hole.

But I’ll give this one to you on account of not caring.

So even though biweekly is universally understood to mean every two weeks, bimonthly can mean every two months or twice a month

I was today years old when I discovered this...and immediately determined it was really stupid.

Many people are puzzled about bimonthly and biweekly, which are often ambiguous because they are formed from two different senses of bi-: "occurring every two" and "occurring two times." This ambiguity has been in existence for nearly a century and a half and cannot be eliminated by the dictionary. The chief difficulty is that many users of these words assume that others know exactly what they mean, and they do not bother to make their context clear. So if you need bimonthly or biweekly, you should leave some clues in your context to the sense of bi- you mean. And if you need the meaning "twice a," you can substitute semi- for bi-. Biannual and biennial are usually differentiated.

I’ll go with whatever Lefty Guns says

bi mon sci fi con.

This thread is the dime store version of the "How Many Days in a Week" thread from the bodybuilding forums.

Or the recent debate whether a week officially starts on Sunday or Monday.

This is most often tied to payroll. Even if we can agree on definition of Bi-Weekly being every other friday and semi-monthly being 2 times a month...many cannot grasp how this results in different payroll calculations.

I've gone through this with my wife at least 5 times and it's gone nowhere.
"Bi-Weekly / Every other Friday" is how you get paid. It's 26 paychecks a year, so the per paycheck amount is slightly less, but you get 2 extra pay dates in a calendar year. "Semi-Monthly is the 15th and last day of the month....24 paychecks a year. So no, I do get paid every other Friday for the 100th time!"

Or the recent debate whether a week officially starts on Sunday or Monday.