Theta
Key Theta in Heston Model refers to the value of the parameter θ in the SDE for the squared asset price volatility χ: dχ = κ(θ-χ)dt + ξσdω
It represents the long variance, or long run average price variance.
As t tends to infinity, the expected value of squared volatility tends to θ
A negative or zero value only serves as an instruction to the pricing routine to calculate it by taking the square of a suitable market implied vol.