To stamp a ticket usually means to validate it or to show a starting time. So, for example, if you have a 24-hour local transport pass/ticket, it needs to be stamped by a person or a machine when you take your first trip so that the authorities know when the 24 hours have expired. The same may be true of parking garages so the attendant knows how long your car has been there and what s/he should therefore charge you. Passports, of course, are stamped when you enter a country.

3. vt. 把（抵用券、赠券等）兑换 http://www.nciku.com/search/en/detail/redeem/1711053 5. vt. [+passport] [ visa etc] 盖章于 to bear the stamp of sth [fig] 有某物的特征 to stamp a mark/word on sth 在某物上盖印/字 http://www.nciku.com/search/en/detail/stamp/1712976