What term describes a fossil formed by compression and flattening?

A fossil formed by compression and flattening is called a compression fossil.

Compression fossils form when an organism, especially a plant, is buried in sediment and pressed flat by the weight of material deposited above it. Fine-grained environments such as ponds, lagoons, river deltas, and floodplains are especially favorable because mud can cover delicate remains quickly.

Leaves are commonly preserved as compression fossils because their naturally flat shape survives flattening relatively well. The fossil usually records the organism's outline and surface features, while its internal anatomy is lost. Shale and clay are among the most common rocks containing these fossils.

Compression fossils are often confused with impressions, molds, or carbon fossils. A compression fossil may retain a thin layer of the original organic material, whereas an impression is generally only a surface mark. When a fossil-bearing rock splits into two matching pieces, the halves are called the part and counterpart, or slab and counter slab.