Definition - What does Heat Spreader mean?
A heat spreader refers to an object that has high thermal conductivity and that is used as a bridge between a heat source and a heat exchanger. It is meant to fully utilize a secondary heat exchanger where the geometry of two surfaces is not the same or fully compatible. A heat spreader can be as simple as a thin copper plate.
Techopedia explains Heat Spreader
A heat spreader essentially “spreads out” the heat from its source, making it possible to sink heat by distributing it to the next level (usually called a heat exchanger). It has high thermal conductivity and heat capacity. A heat spreader is usually needed in an electronic system where the heat source has a high flux density and the heat cannot directly sink to the secondary heat exchanger; efficiency must thus be added to the system, such as those that are air-cooled, which have a lower heat transfer coefficient than liquid-cooled ones. A heat spreader is used in electronic devices with powerful processing units and electrical or mechanical components.