JOSS (acronym for JOHNNIAC Open Shop System) was one of the first interactive, time-sharing programming languages. It pioneered many features that would become common in languages from the 1960s into the 1980s, including use of line numbers as both editing instructions and targets for branches, statements predicated by boolean decisions, and a built-in source-code editor that can perform instructions in direct or immediate mode, what they termed a conversational user interface.

JOSS was initially implemented on the JOHNNIAC machine at RAND Corporation and put online in 1963. It proved very popular, and the users quickly bogged the machine down. By 1964, a replacement was sought with higher performance. JOHNNIAC was retired in 1966 and replaced by a PDP-6, which ultimately grew to support hundreds of computer terminals based on the IBM Selectric. The terminals used green ink for user input and black for the computer's response. Any command that was not understood elicited the response Eh? or SORRY.

The system was highly influential, spawning a variety of ports and offshoots. Some remained similar to the original, like TELCOMP and STRINGCOMP, CAL, CITRAN, ISIS, PIL/I, JEAN (ICT 1900 series), Algebraic Interpretive Dialogue (AID, on PDP-10); while others, such as FOCAL and MUMPS, developed in distinctive directions. It also bears a strong resemblance to the BASIC interpreters found on microcomputers in the 1980s, differing mainly in syntax details.