diff --git a/docs/2a/ece250.md b/docs/2a/ece250.md
index ee72ecb..dc8875c 100644
--- a/docs/2a/ece250.md
+++ b/docs/2a/ece250.md
@@ -158,3 +158,7 @@ fn sort(A: Vec) {
     - A **connected graph** is such that there exists a path from any node in the graph to any other node.
     - A **connected component** is a subgraph such that there exists a path from any node in the subgraph to any other node in the subgraph.
     - A **tree** is a connected graph without cycles.
+
+### Directed acyclic graphs
+
+a DAG is acyclic if and only if there are no **back edges** — edges from a child to an ancestor.