Hello,
As an executive recruiter, I am considering 'making up the difference' of the bonus that a candidate will not receive, if he accepts a position with a client of mine. Is my payment to the candidate as the recruiter, considered to be by tax definition a Sign On Bonus?
Thank you.

Yes. It would be considered compensation to the client; you would originate a Form 1099-MISC for him to report the income to the IRS if it is more than $600.