Anisotropy: Anisotropy is when a material's properties differ by direction; 3D printed parts are often weaker across their layers than along them.
A printed part is built from bonded layers, and the bond between layers is rarely as strong as the material within a layer. FDM parts show this most clearly: pull along the layers and the plastic carries the load; pull across them and the part can split along a layer line. Powder bed and resin parts are much closer to equal in every direction, though not perfectly. Machined and moulded parts are generally treated as the same in every direction, apart from effects such as grain in metal or fibre direction in filled plastics.
What it means for the price
Anisotropy reaches the price through orientation. The strongest way to print a loaded part may make it taller, need more supports or print slower than the cheapest way, so a shop may override the cheapest orientation for a functional part. A buyer ordering structural parts should state the load direction, so the quote is made for the orientation that will actually be printed.
Example: a hook printed lying flat carries its load along its layers; the same hook printed standing up may snap at the bend. The figures are an example.
Common confusions
Anisotropy is not porosity, though both reduce strength. Thinner layers do not cure it. And it is not warping, which is about shape, not strength. Read design for 3D printing.
Go deeper: Design for 3D printing: walls, overhangs, holes, clearances and orientation