Acidophilus

Acidophilus is a general name for a group of probiotics, often added to milk or sold as a capsule, which contains one or more of the following bacteria that aid in digestion:

Lactobacillus acidophilus (A)
Lactobacillus casei (C)
Lactobacillus bulgaricus (L)
Bifidobacterium species (B)
Streptococcus thermophilus

Only L. acidophilus is the true acidophilus species, but many producers (mainly in the US) use it as a more generic name for mixtures of bacteria, one of which is L. acidophilus.

See the individual bacteria and the link on probiotics for more detailed information.