Cassette tape.
"As the transistor radio defined small music in the 1960s, the portable CD player in the 1990s, and the MP3 player in the 2000s, so did the Walkman define very small portable music in the 1980s, with cassette sales overtaking those of LPs. Total vinyl record sales remained higher well into the 1980s due to greater sales of singles, although cassette singles achieved popularity for a period in the 1990s."