Crying
Crying, sobbing, or to cry or sob, was the welling or shedding of tears, produced by tear ducts, in the eyes, as an emotional response to pain or sadness. 

An observation made by the Changeling Odo was that "when a humanoid cries, the epidermis below the eyes swells noticeably." ()

Following the assassination of Klingon Chancellor Gorkon and the naming of his daughter Azetbur as his successor, Montgomery Scott was willing to bet that "that Klingon bitch killed her father."  He cited the fact that "They don't place the same value on life that we do," adding that following his death, "She did not shed one bloody tear." Spock, however, remained unconvinced, stating that his observation was "hardly conclusive [...] as Klingons have no tear ducts." ()

In expressing his desire to be Human to Q in 2366, Data explained that though he had the "curiosity of Humans [...] there are questions I will never have the answer to. What is it like to laugh, or cry, or to experience any Human emotions." () Four years later, after receiving an emotion chip, Data was finally able to cry after finding his missing cat Spot. ()

After experiencing hyper-evolution in 2372, Tom Paris began lamenting his own death to The Doctor, expressing how he saw himself having a "big funeral with lots of pretty girls all crying. Except Torres. Torres doesn't cry. Did you ever notice that? I don't trust people who don't cry. Of course, my father, he'd say crying is a sign of weakness. I never believed that." When he asked in The Doctor cried, The Doctor explained that it was not in his program, and Paris felt that that was a shame. Paris further lamented that "You know, it's funny. What I remember most about being a kid are the times I spent in my room crying." () 

Ironically, in the case of The Doctor, sometime prior to 2377, Lieutenant Torres made a technological enhancement to his emotional subroutines, so that he could "truly appreciate" Puccini's Tosca. He explained that "I sobbed through the entire third act." () 

In 2377, Seven of Nine experienced crying for the first time, following the departure of three former Borg drone children, Azan, Rebi, and Mezoti. After Icheb (the fourth former Borg drone who remained aboard ) noted that she was crying, Seven responded that she believed that her "ocular implant must be malfunctioning." While giving Seven a checkup, The Doctor informed her that "Crying is nothing to be ashamed of," later adding that "Saying goodbye to the children was a traumatic experience. Crying was a normal response." He eventually found that the result of her crying was a "glitch in [her] primary cortical node." Later, after Icheb saved Seven's life, he noted that her ocular implant was malfunctioning again, after witnessing her tears, however, The Doctor's scans indicated that it was "functioning perfectly," proving that her tears were genuine. ()

See also 
 Battle cry
 Rallying cry
 "Whales Weep Not!"

External link 
 
Physiology