The words pollenizer (polleniser) and pollinator are often confused. A pollenizer is the plant that provides pollen. A pollenizer is always a plant. A pollinator is the biotic agent that moves the pollen, whether it be bees, moths, bats, or birds, etc.
The verb form to pollenize, is to be the source of pollen, or to be the sire of the next plant generation.
While some plants are capable of self pollenization, the term is more often used in pollination management as a plant that provides abundant, compatible, and viable pollen at the same flowering time as the pollenized plant. For example most crabapple varieties are good pollenizers for any apple variety that blooms at the same time, and are often used in apple orchards for the purpose. Some apple cultivars produce very little pollen; some produce pollen that is sterile, or incompatible with other apple varieties. These are poor pollenizers.
Plants are sometimes mistakenly called pollinators. For example, some nursery catalogs may say variety X should be planted as a pollinator for variety Y, when they actually should be referring to it as a pollenizer. Strictly, a plant can only be a pollinator when it is self fertile and it physically pollinates itself without the aid of an external pollinator, as in the case of apomictic species like some rowans and hawthorns.
See also: Pollination