Short Definition
Space complexity describes how much memory an algorithm needs as input size grows.
Intuition
A fast method can still fail if it needs too much memory.
Technical Definition
Space complexity is the asymptotic memory required by an algorithm, including auxiliary data structures and sometimes input storage depending on convention.
Example
A dynamic programming algorithm may trade extra memory for faster computation.
Common Misunderstandings
Low time complexity does not imply low memory use.
Memory access patterns can matter as much as total memory volume.