For the cytoplasmic extension present during apoptosis please see Bleb (cell biology)

In medicine, bleb is a large blister filled with serous fluid. Blebs can form in a number of tissues due to different pathologies, including frostbitten tissues, and as a cause of spontaneous pneumothorax. In the lungs, a bleb is a collection of air within the layers of the visceral pleura. In ophthalmology, blebs may be formed intentionally in the treatment of glaucoma.

In geology, mineralogy, petrology, and related geochemical sciences, bleb is a noun or descriptive term meaning the inclusion of a mineral within a larger mineral that is bubble-like. Commonly used for small inclusions, such as ones tens of micrometres in size. An example usage in the literature is a bleb of sylvite in chlorite.