Almost never stands alone - it's the first half of 天安门 (Tiananmen, 'Gate of Heavenly Peace'), the Ming-era gate at the north end of Tiananmen Square. Also used as a company name (e.g. 天安保险, 天安财险).
history
The name 天安门 was bestowed in 1651 under the Shunzhi Emperor; before that it was called 承天门 ('gate that upholds heaven'). The current gate survived multiple rebuilds after fires and earthquakes.
