Breasts was the collective term for physical protrusions of mammary glands on most mammalian species, both male and female. In most humanoid species, females had larger breasts, which caused them to be easily distinguished from males. While breasts were useless on most males, females typical could use them for breast feeding and sexual attraction. (TNG: "A Matter Of Honor"; VOY: "Deadlock")
Most cultures in the known Galaxy required women to cover their breasts in public, and it was considered shameful to appear in public with them exposed. The exception to this rule was Betazoid wedding ceremonies, in which all attendees were naked. (TNG: "Cost of Living"). If for some reason, a woman was made to show her breasts in public due to loss of clothing, they would typically use an arm to cover their breasts until clothing could be obtained.
Such was the case when the Enterprise NX-01 was captured and impounded by Suliban Cabal. The crew used their room intercoms to formulate a plan to escape. The only escape route was through the air vents and Ensign Hoshi Sato was small enough to fit. When she made her way to the outside of Lieutenant Malcolm Reed's room, her shirt was caught on a hook and she tried to get free, but ended up losing her shirt while sliding through. When she opened Reed's door, she held her hands over her breasts and told him to not say anything and to fetch her a shirt. (ENT: "Shockwave, Part II")Until the reforms of Zek, Ferengi females were required to not wear clothing. They also were prohibited from leaving their homes, due to the taboo of a Ferengi female appearing nude in public. In order to get around this, many Ferengi females would tightly wrap their breasts and pretend to be male in order to acquire profit and to travel away from Ferenginar. Such was the case of Pel in 2370 who in addition to wrapping her breasts, also enhanced her lobes in order to leave Ferenginar and work at Quark's Bar. (DS9: "Rules of Acquisition") Additionally, in order to fill in for the ill Ishka, working on Zek's behalf, to prove the value of females to Nilva an influential FCA commissioner, Quark goes through cosmetic surgery and poses as a female Ferengi named Lumba. (DS9: "Profit and Lace")
"His eyes should flash,
and his breast protrude,
And this should be his customary attitude."
While on the Ba'ku planet, Deanna Troi asked Beverly Crusher if she had noticed how her "boobs" had started to firm up, as a result of the metaphasic radiation that affects that planet, to which Crusher replied, "Not that we care about such things in this day and age," though Troi's snickering reaction suggested otherwise. Data, who overheard this conversation, repeated it to Worf. (Star Trek: Insurrection)