Thelarche /Î¸É¨ËlÉrkiË/ is the onset of secondary (postnatal) breast development, usually occurring at the beginning of puberty in girls. Its etymology is from Greek Î¸Î·Î»Î® [tÊ°elá¸], ânippleâ and á¼ÏÏÎ® [arkÊ°á¸], âbeginning, onsetâ.