diff --git a/lectures/complex_and_trig.md b/lectures/complex_and_trig.md
index bdeb2ec4..002a888b 100644
--- a/lectures/complex_and_trig.md
+++ b/lectures/complex_and_trig.md
@@ -45,6 +45,8 @@ So let's dive in.
 
 A complex number has a **real part** $x$ and a purely **imaginary part** $y$.
 
+Here, $i$ denotes the imaginary unit, satisfying $i^2 = -1$.
+
 The Euclidean, polar, and trigonometric forms of a complex number $z$ are:
 
 $$