The simple answer is that Issan is the Northeast area of Thailand bordered by Laos and the Mekong in the North and East, Cambodia in the Southeast, the Pratchinburi mountains in the South and Phetchabun mountain range to the North and Central plains. The word Issan comes from the Sanskrit language meaning Northeast.
The harder answer is just why the Northeast region of Thailand is called Issan. You will find many arguments across the internet about royal Lao, Khmer, Chinese, Mon and Thai bloodlines that will leave you even more confused than when you started. The simple version is that a long time ago before Thailand ( Siam ) existed many people inhabited this region from Laos and Cambodia and their descendants are still there to this day.
The Issan region of Thailand can be seen almost as another country in and of itself. The primary language of Issan is Issan, which is a combination of Lao and Khmer dialects using the Thai alphabet. The dialect will change depending where you are in Issan. Along the border areas of Laos and the along the Mekong the dialect will be heavily Lao influenced whereas the further you get South and into the central areas the dialect will still be primarily Lao but with more Khmer thrown in. In Nakhon Phanom I hear a lot of Lao and Thai mixed together while in Mukdahan you get Lao, Thai and a little Khmer for good measure.
Just as the language is distinguishably different from the rest of Thailand the food also takes on a different taste and that is definitely of Lao heritage. You’ll hear many people claim that Issan food is just a regional Thai cuisine…I would bet they don’t say such things out loud while in Issan. While many Issan dishes have filtered down throughout Thailand with its people the dishes themselves can never be categorized as Traditional Thai food. While Thai food can be very spicy, Issan food redefines spicy. The food is generally more salty, spicy and sour. You won’t find many sweet dishes here.
Issan has gone through periods of Thaianization since the early days of the last century but the area has largely remained true to it’s conglomerate heritage. Although there have been times when the powers that be have strived to make Issan more Thai they still recognized the region by officially naming it Issan early on in the 20th century.
All this and I have still only scratched the surface of what Issan truly is. My hope is to add more to this topic in the future with the knowledge I learn first hand travelling throughout Issan and spending a great deal of time in areas such as Mukdahan and Nakhon Phanom.