But that commercial meaning has all but fallen away.

“Citgo’s the last thing you think of,” said Leverett Ball, 23, a radio promotions assistant who was standing with his father, the economist Laurence Ball, near the sign on a recent evening.

“I think of it as a symbol of the Red Sox,” the elder Mr. Ball said.

The sign has inculcated itself into the consciousness of this sports-crazed city because it is close to both Fenway Park and the finish line of the Boston Marathon. What is more, it is a useful flare in a city that is notoriously difficult to navigate.

Some of its supporters have also argued that it has artistic significance.

“Beyond the fact that it’s an orienting beacon, that it’s a bull’s-eye for the home run of the Red Sox, that it’s a symbol for the marathon finish line, is that it is a wonderful piece of American Pop Art from the 1960s,” said Arthur Krim, a member of the preservation faculty at Boston Architectural College, who tried unsuccessfully to win the sign a landmark designation in the early 1980s.

Boston has other memorable structures. The glass monolith formerly known as the Hancock Tower, for example, is the city’s tallest building. And the Bunker Hill Monument rises above the Charlestown neighborhood. But none of those instantly convey “Boston” to a global audience in the way that the Statue of Liberty or the Empire State Building or the Chrysler Building shout “New York.”

But the Citgo sign does. And it does it in a unique way, supporters said.

“In a city as architecturally conservative as Boston, the Citgo sign represents a departure from the banal,” wrote Anulfo Baez, a local art writer, on his blog Evolving Critic, “and we’ll be better off by saving it for future generations of Bostonians to enjoy.”

The sign has not always been so beloved. With the nation in the grips of an energy crisis, the state asked Citgo in 1979 to let it go dark, and it stayed that way for four years — much to the chagrin of Robert Campbell, the architecture critic of The Boston Globe, who called it “the crown jewel of the Boston skyline” and “the best symbol Boston owns of a whole era in American history, now drawing to a close: the Age of Abundance.”

The sign was lit again in 1983, as speakers blasted the song “You Light Up My Life,” and it has stayed that way nearly every night since. In 2010, the sign was fitted with 9,000 LED bulbs.