discount-rate
where 




r

t




{\displaystyle r_{t}}

 is the reward at step 



t


{\displaystyle t}

, 



γ
∈
[
0
,
1
)


{\displaystyle \gamma \in [0,1)}

 is the discount-rate. Gamma is less than 1, so events in the distant future are weighted less than events in the immediate future.
