The process of fetching the value of a texture at a particular texture coordinate, potentially involving combining multiple texel values together.
Filtering can happen in two directions: magnification and minification. Magnification happens when the fragment area projected into a texture is smaller than the texel itself. Minification happens when the fragment area projection is larger than a texel.
Texture filtering where the texel closest to the texture coordinate is the value returned.
Texture filtering where the closest texel values in each dimension of the texture are access and linearly interpolated, based on how close the texture coordinate was to those values. For 1D textures, this picks two values and interpolates. For 2D textures, it picks four; for 3D textures, it selects 8.
Subimages of a texture. Each subsequence mipmap of a texture is half the size, rounded down, of the previous image. The largest mipmap is the base level. Many texture types can have mipmaps, but some cannot.
Texture filtering that uses mipmaps. The mipmap choosen when mipmap filtering is used is based on the angle of the texture coordinate, relative to the screen.
Mipmap filtering can be nearest or linear. Nearest mipmap filtering picks a single mipmap and returns the value pulled from that mipmap. Linear mipmap filtering pics samples from the two nearest mipmaps and linearly interpolates between them. The sample returned in either case can have linear or nearest filtering applied within that mipmap.
Texture filtering that takes into account the anisotropy of the texture access. This requires taking multiple samples from a surface that covers an irregular area of the surface. This works better with mipmap filtering.
Functionality that is not part of OpenGL proper, but can be conditionally exposed by different implementations of OpenGL.