Complex number consists of two parts in its expression a + ib.
"a" is the real part and "ib" is the imaginary part of the complex number.
In most mathematics study materials, the "i" represents the imaginary part.
But, for electrical and electronics engineering studies, the "i" notation is replaced by the letter "j". Why?
It is because in electrical or electronics engineering, the symbol or notation for "electric current" is also "i" which will cause confusion. Thus in these two engineering disciplines, the imaginary part of the complex number is replace by "j".
Complex number a + ib therefore has the same meaning as a + jb, depending on where it is used.
Bonus information: "i" itself also represents direction, and is 90 degree away from the real component(term) of complex number.