ANSI C is the first official standard for the C programming language. The official Rationale published by the standards committee (WG14 document N897) states that the work produced “the original ANSI Standard (ANSI X3.159-1989, the so-called ‘C89’).” This version is the one most people mean by the term “ANSI C.”
The same Rationale records that “C89 was quickly adopted as an International Standard (ISO/IEC 9899:1990, commonly called ‘C90’), with changes to clause and subclause numbering to conform to ISO practices.” In other words, C89 and C90 describe essentially the same language; the ISO version mainly renumbered the sections. The ISO catalogue record for ISO/IEC 9899:1990, “Programming languages - C,” confirms the standard was published in 1990 and later given withdrawn status as newer editions superseded it.
According to the Rationale, the committee’s charter was to codify common existing practice rather than to invent a new language. It notes that “the vast majority of the language defined by C89 was precisely the same as defined in Appendix A of the first edition of The C Programming Language by Brian Kernighan and Dennis Ritchie,” the book commonly called “K&R.” The standard turned that informal reference into a precise, agreed definition.
Standardization mattered because it let compiler writers and programmers rely on a single, portable definition of C across different machines and vendors. ANSI C / C90 became the baseline that later revisions of the language, such as C99 and beyond, built upon.