Searches on its website for the word have spiked by 1,740% in 2022, according to the company.
Gaslighting is the act or practice of grossly misleading someone, especially for one's own advantage.
"In this age of misinformation - of 'fake news', conspiracy theories, Twitter trolls, and deep fakes - gaslighting has emerged as a word for our time," Merriam-Webster said in a statement on Monday.
Curiously, search interest in the word was not driven by any single event, the company's editor-at-large told the Associated Press.
"It was a word looked up frequently every single day of the year," Peter Sokolowski told the US wire service.