Seija (pr. say-yah; Forvo) is a Finnish female name meaning “tranquil, serene” from Finnish seijas (tranquil, serene, calm, clear). It’s a fairly modern name in Finland, first invented by Yrjö Karilas, a Finnish writer who was inspired to create the name from Seia, the name of a Roman goddess of agriculture who protects the seeds sown into the earth, and combining it with the Finnish word seijas, which he used for his daughter in 1917. In 1924, a family friend wrote a book called The Story of Seija-girl, which made the name more popular.
Seija is also a Spanish surname, a variant of Ceja, which is a locational name for someone who came from several towns called Ceja. In Spanish Ceja means “eyebrow”, though in this case it’s possible that the name denotes a characteristic of the area- like the summit of a hill.
I also came across the name as belonging to a female character in a Japanese game; her name apparently means “right and wrong” or “good and evil” written with the kanji 正邪, though it seems to have been created specifically for the character in this game and isn’t used as an actual name in Japan.
Origin: Proto-Finnish, Proto-Indo-European