Fractional Kelly
Fractional Kelly means staking a fixed fraction of the amount the Kelly formula suggests, such as a half or a quarter.
In plain English
It trades some theoretical growth for a large reduction in volatility, and it hedges against the probability estimate being wrong. Since real estimates usually are somewhat wrong, the fractional version is what most careful practitioners actually use.
Example
If the formula suggests four percent of bankroll, quarter Kelly stakes one percent.
Why it matters
It is the practical form of the idea, and the version that acknowledges your inputs are estimates rather than facts.
How V8 uses this term
Taught as a concept only. V8 does not compute a stake for any user.