"The present data based on ultrasonographic bladder monitoring and biochemical analyses indicate that squirting is essentially the involuntary emission of urine during sexual activity, although a marginal contribution of prostatic secretions to the emitted fluid often exists." (Salama et al, 2015)
References
Salama, S., Boitrelle, F., Gauquelin, A., Malagrida, L., Thiounn, N., & Desvaux, P. (2015). Nature and Origin of “Squirting” in Female Sexuality. The Journal of Sexual Medicine, 12(3), 661–666.
https://doi.org/10.1111/jsm.12799