Cyst

Definition:

A lump filled with either fluid or soft material, occurring in any organ or tissue; may occur for a number of reasons but is usually harmless unless its presence disrupts organ or tissue function.

Source: MCG Otolaryngology Glossary 

More:

Cysts are also small microorganisms generally consisting of one or a small number of living cells. Examples are giardia lamblia and cryptosporidium.

Related words:

Benign - Lesion - Sebaceous cyst(s)