more or less, yep.

Bicentennial = 200 years (two centennials, or two one-hundred years)
Semicentennial = 50-year anniversary (Oklahoma celebrated this for their statehood in 1957)
Biannual = 2 years, like some obscure races (human powered submarine, for one)
Semiannual = every 6 months (see your local discount furniture store sales for examples)
Bimonthly = every 2 months (like your girlfriend's period in Jr. high school)
Semimonthly = every half a month (many employers use this to ensure exactly 24 pay periods)
Biweekly = every 2 weeks (many other employers pay on this schedule for 26 pay periods per year)
Semiweekly = twice per week (some Ye Olde Tyme newspapers did this a lot, as news wasn't all that easy to come by for a daily paper back then)

I can't believe I'm contributing to this. Hopefully it dies here.