Given a line segment $AB$ in the plane, find all possible points $C$ such that in the triangle $ABC$, the altitude from $A$ and the median from $B$ have the same length.
1. **Define the problem and setup:**
   Given a line segment \( AB \) in the plane, we need to find all possible points \( C \) such that in the triangle \( ABC \), the altitude from \( A \) and the median from \( B \) have the same length.

2. **Define midpoints and circles:**
   Let \( M \) be the midpoint of \( AC \) and \( N \) be the midpoint of \( AB \). Let \( O \) be the vertex of the equilateral triangle \( NBO \).

   Define the following circles:
   - \( o_1 \) with center \( N \) and radius \( NB \)
   - \( o_2 \) with center \( O \) and radius \( OB \)
   - \( o_3 \) with center \( B \) and radius \( BM \)

3. **Consider point \( C \) such that \( M \) lies on circle \( o_2 \):**
   Let \( P \) be the intersection of \( o_1 \) and \( o_3 \) on the other side of \( AB \) than \( C \). 

4. **Analyze the properties of \( P \):**
   Since \( AP \) is tangent to \( o_3 \), we have \( BP \perp AP \). Given that \( \triangle BON \) is equilateral, \( \triangle BMP \) is also equilateral. Therefore, the altitude of \( \triangle BMP \) from \( M \) passes through \( N \), implying \( MN \perp BP \).

5. **Relate \( AP \) and \( BC \):**
   Since \( MN \perp BP \), it follows that \( AP \parallel BC \). Thus, \( BP \) is the length of the altitude of \( \triangle ABC \) from \( A \). Given \( BP = BM \), our point \( C \) satisfies the necessary conditions.

6. **Determine the set of points \( C \):**
   For a given altitude from \( A \), there is only one "good" point \( C \) for each side of line \( AB \). Therefore, the requested set of points \( C \) is a circle \( o \) homothetic to \( o_2 \) with respect to \( A \) in scale \( k = 2 \) and a circle symmetric to \( o \) with respect to \( AB \) (excluding point \( B \)).

The final answer is the set of points \( \boxed{ C } \) forming a circle homothetic to \( o_2 \) with respect to \( A \) in scale \( k = 2 \) and a circle symmetric to \( o \) with respect to \( AB \) (excluding point \( B \)).