Lo-fi (also typeset as lofi or low-fi) is a music or production quality in which elements usually regarded as imperfections in the context of a recording or performance are present, sometimes as a deliberate stylistic choice. The standards of sound quality (fidelity) and music production have evolved over the decades, meaning that some older examples of lo-fi may not have been originally recognized as such. Lo-fi began to be recognized as a style of popular music in the 1990s, when it became alternately referred to as DIY music (from "do it yourself"). Some subsets of lo-fi music have become popular for their perceived nostalgic or relaxing qualities, which originate from the imperfections that define the genre.

Traditionally, lo-fi has been characterized by the inclusion of elements normally viewed as undesirable in most professional contexts, such as misplayed notes, environmental interference, or phonographic imperfections (degraded audio signals, tape hiss, and so on). Pioneering, influential, or otherwise significant artists and bands include the Beach Boys (Smiley Smile and Wild Honey), R. Stevie Moore (often called "the godfather of home recording"), Paul McCartney (McCartney), Todd Rundgren, Lee Scratch Perry, Peter Ivers, Jandek, Daniel Johnston, Neutral Milk Hotel, Guided by Voices, Sebadoh, Beck, Pavement, and Ariel Pink.

Although "lo-fi" has been in the cultural lexicon for approximately as long as "high fidelity", WFMU disc jockey William Berger is usually credited with popularizing the term in 1986. At various points since the 1980s, "lo-fi" has been connected with cassette culture, the DIY ethos of punk, primitivism, outsider music, authenticity, slacker/Generation X stereotypes, and cultural nostalgia. The notion of "bedroom" musicians expanded following the rise of modern digital audio workstations, leading to the invention of the nearly synonymous term bedroom pop. In the late 2000s, lo-fi aesthetics served as the basis of the chillwave and hypnagogic pop music genres. The 2010s saw the emergence of the chillout-influenced lo-fi hip hop style, which gained widespread popularity on YouTube.

Lo-fi is the opposite of high fidelity, or "hi-fi". The perception of "lo-fi" has been relative to technological advances and the expectations of music listeners, causing the rhetoric and discourse surrounding the term to shift numerous times throughout its history. Usually spelled as "low-fi" before the 1990s, the term has existed since at least the 1950s, shortly after the acceptance of "high fidelity", and its definition evolved continuously between the 1970s and 2000s. In the 1976 edition of the Oxford English Dictionary, lo-fi was added under the definition of "sound production less good in quality than 'hi-fi'". Music educator R. Murray Schafer, in the glossary for his 1977 book The Tuning of the World, defined the term as "unfavourable signal-to-noise ratio."

At its most crudely sketched, lo-fi was primitivist and realist in the 1980s, postmodern in the 1990s, and archaicist in the 2000s.

There was virtually no appreciation for the imperfections of lo-fi music among critics until the 1980s, during which there was an emergent romanticism for home-recording and "do-it-yourself" (DIY) qualities. Afterward, "DIY" was often used interchangeably with "lo-fi". By the end of the 1980s, qualities such as "home-recorded", "technically primitive", and "inexpensive equipment" were commonly associated with the "lo-fi" label, and throughout the 1990s, such ideas became central to how "lo-fi" was popularly understood. Consequently, in 2003, the Oxford Dictionary added a second definition for the term—"a genre of rock music characterized by minimal production, giving a raw and unsophisticated sound".

The identity of the party or parties who popularized the use of "lo-fi" cannot be determined definitively. It is generally suggested that the term was popularized through William Berger's weekly half-hour radio show on the New Jersey–based independent radio station WFMU, titled Low-Fi, which lasted from 1986 to 1987. The program's contents consisted entirely of contributions solicited via mail and ran during a thirty-minute prime time evening slot every Friday. In the fall 1986 issue of the WFMU magazine LCD, the program was described as "home recordings produced on inexpensive equipment. Technical primitivism coupled with brilliance."

A third definition was added to the Oxford Dictionary in 2008: "unpolished, amateurish, or technologically unsophisticated, esp. as a deliberate aesthetic choice." In 2017, About.com's Anthony Carew argued that the term "lo-fi" had been commonly misused as a synonym for "warm" or "punchy" when it should be reserved for music that "sounds like it's recorded onto a broken answering-machine".