definition 

A taxable transfer of assets from a traditional IRA, SEP, or SIMPLE IRA to a Roth IRA.

commentary 

If is is assumed that the current tax rate at which the transfer will be taxed is lower than the tax rate at which the future withdrawals from the traditional plan would have been taxed,  then a conversion can be a good choice.   Since conversions are taxed currently, they eliminate the risk of a increased future tax rates due to changes in the Internal Revenue code.  Conversions can be transferred back to the traditional plan through the use of Roth Recharacterization.