Wiktionary
n. 1 (context geology uncountable English) The rock type, resembling pillows, typically formed when lava emerges from an underwater volcanic vent or a lava flow enters the ocean. 2 (context geology countable English) A rock of that type.
WordNet
n. lava that hardened in rounded shapes suggestive of pillows; believed to result from underwater eruptions
Wikipedia
Pillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava under water, or subaqueous extrusion. Pillow lavas in volcanic rock are characterized by thick sequences of discontinuous pillow-shaped masses, commonly up to one metre in diameter. They form the upper part of 'Layer 2' of normal oceanic crust.
Usage examples of "pillow lava".
As the town had grown westward new tents had attached to the previous ones like lobes of pillow lava.
A pillow lava hill had crumbled, the signal to a waldo had been interrupted, the waldo arm hadn't shut off.