曾 is the literary/short form of 曾经 (céngjīng, once/formerly). In modern speech, 曾经 is more common. Also read as zēng in 曾祖父 (zēngzǔfù, great-grandfather).
grammar
曾 indicates a past experience, similar to the 过 (guò) particle but more formal: 曾去过 = once went.
