The albedo map defines the base RGB color of each texel. It is configured using a texture.txt file which is directly referenced from the material (usually within a mesh file although sometimes from the asset's config.txt directly).
The albedo map is the Physically Based Rendering equivalent of the diffuse map used in legacy materials. At a casual glance, these fill the same role and the terms can be interchanged, however to be strictly accurate the albedo is the surface color irrespective of any lighting effects, whereas the diffuse map frequently included burned-in lighting (pre-baked ambient occlusion, etc.) The albedo map should never include pre-baked lighting, as that would interfere with the correct operation of the PBR equations.
Unless otherwise specified by the material in use, the RGB channels use the sRGB color space.
It is common for (semi-)transparent materials to store the alpha blend factor (ie. opacity level) in the albedo map's alpha channel. Materials which are fully opaque may use the albedo's alpha channel for some other purpose, or not at all.
If no alpha channel is specified, the outcome is identical to a fully white (1.0) alpha channel.
Some legacy material types offer automatic selection of alpha blending, alpha masking or opaque rendering modes based on the contents and configuration of the albedo map. This introduces uncertainties when texture replacement is used as to what material configuration will result. TRS18 materials do not attempt this, and instead rely on the content creator to select the appropriate material type.
TBD: See Normal map for examples.