The term “Spanish guitar” is what we nowadays call a ”classical guitar.” The guitar as we know it now was developed in Spain close to three hundred years ago. The classical guitar came to America, and later Hawaii, along with the Spanish explorers and sailors. It was smaller than the typical acoustic model of today and the strings were made from gut (nylon in the present day). Nowadays people use the term “Spanish guitar” and “classical guitar” almost interchangeably, although it more often than not describes the music the guitar is playing rather than the guitar.
The Hawaiian guitar, is just as confusing a term. Believe it or not, Hawaii was apparently the birthplace of the steel string guitar!
Presently, if only to be more confusing, most people think of any guitar that you play on your lap to be a Hawaiian guitar. “Lap steel guitar” is also a typical name for it. Lap steel guitars look like short, stumpy pedal steel guitars but without the pedals.
But in Hawaii, a “Hawaiian guitar” is any guitar played in “slack-key” style. Slack-key is, essentially slide guitar done on an instrument with lowered (hence “slack”) tuning. Traditionally Open G is the tuning of choice.