a dialect of Middle English
"The scholar studied the southwestern dialect of Middle English to understand its unique vowel shifts."
situated in or oriented toward the southwest
"The house has windows facing southwestern, allowing warm sunlight to fill the room throughout the afternoon."
of a region of the United States generally including New Mexico; Arizona; Texas; California; and sometimes Nevada; Utah; Colorado
"The southwestern desert features unique landscapes found in states like New Mexico, Arizona, and Texas."
Of or pertaining to the southwest.
"The southwestern corner of the room was filled with warm sunlight."
In plain English: Southwestern describes something that comes from or is located in the southwest part of a place, like the American Southwest region known for its desert landscapes and unique culture.
"The southwestern corner of my garden gets plenty of sunlight during winter."
The word southwestern comes from the combination of the words south and west. It has been used in English since at least the 14th century to describe anything located between or toward those two directions.