Areal differentiation is a foundational concept in geography, emphasizing the unique character and distinctiveness of different places on the Earth's surface. Articulated prominently by Richard Hartshorne, it posits that the primary task of geography is to describe and interpret the Earth as a mosaic of unique regions, each with its own specific combination of physical and human phenomena. This approach focuses on understanding 'why one area differs from another' and 'how these differences are organized spatially'.

Instead of seeking universal laws (a nomothetic approach), areal differentiation adopts an idiographic perspective, aiming to understand the particularity and uniqueness of places. Geographers employing this concept analyze the complex interplay and interrelationships of various elements within a specific area – such as climate, landforms, vegetation, soil, population distribution, economic activities, cultural practices, and political structures. The synthesis of these diverse elements creates the distinctive identity and character of a region.

For instance, a geographer studying a particular valley would not just describe its climate or its agriculture in isolation, but would examine how the climate influences the types of crops grown, how the topography affects settlement patterns, and how historical human activities have shaped the current landscape and culture. All these factors, interacting dynamically, contribute to the valley's unique 'areal differentiation' from other valleys or regions.

This concept highlights the holistic and integrative nature of geography, demonstrating how the discipline synthesizes knowledge from various systematic branches (like climatology, geomorphology, economic geography) to provide a comprehensive understanding of specific places. It underscores that while systematic geography provides the building blocks, areal differentiation is crucial for understanding the complex, integrated reality of regions on Earth.