A steel-string acoustic guitar is a modern form of guitar descended from the classical guitar, but strung with steel strings for a brighter, louder sound. It is often referred to simply as an acoustic guitar, although strictly speaking the nylon-strung classical guitar is acoustic as well.

The most common type can be called a flat-top guitar to distinguish it from the more specialized archtop guitar and other variations.

The standard tuning for an acoustic guitar is E-A-D-G-B-E (low to high), although many players, particularly fingerpickers, use alternate tunings (scordatura), such as "open G" (D-G-D-G-B-D), "open D" (D-A-D-F♯-A-D), or "drop D" (D-A-D-G-B-E).