UC Irvine sociologist Rubén Rumbaut, a Cuban American who himself arrived in the United States as a child, wrote in 2004 that he first came cross the term “half-second” generation decades ago in an early twentieth-century volume on Polish immigrants, where it was used to describe foreign-born youths who came of age in the U.S.

“It made an impression on me,” Rumbaut told me by phone. “I came to this country on the eve of my twelfth birthday.” Starting in 1969 and through the 1970s, he used the term “one-and-a-half generation” to describe similar youths in Cuban immigrant families. In the 1980s, while writing about Southeast Asian youths, he switched to the decimal version, “1.5 generation.”

While the term was coined by a Latino, it had special resonance for Asian immigrants, particularly immigrants from Korea. Just as the Japanese have terms to describe first, second and third-generation immigrants, Koreans have a unique term for those in between the first and the second generation, which they refer to as ilchom ose. UCLA anthropologist Kyeyoung Park wrote about the term in 1999 for the Amerasia Journal:

More than three-quarters of Korean immigrants are post-1965, many immigrating on family reunification provisions. The Korean immigrant community includes many child immigrants who are often called 1.5ers, or what is called ilchom ose within the Korean American community, the term first used in the 1970s (of both Los Angeles and New York Korean American communities) and popularized by community leaders such as Professor Eui-young Yu and Bong Hwan Kim, former director of the Korean Youth and Community Center. In the early 1980s, It was K.W. Lee, the editor of the Korea Times English Edition, who first wrote about the 1.5 generation Korean Americans.

“1.5 generation” is used by most as a blanket term for those born abroad and raised here. But Rumbaut has sub-categories to describe who arrived at what age: Those who arrived before age five he considers “1.75 generation,” closer to the second, with little or no memory of their native country. Those who arrived between 13 and 17 would be generation 1.25, more likely to have an outlook similar to the first generation.

via Introducing the cultural mashup dictionary: Our first term, 1.5 generation | Multi-American.