A heap table usually refers to a database table whose records are stored without any specific order. It can have a different meaning for different database engines. Use in combination with the appropriate database engine tag.
MS SQL Server: heap
A heap is a table without a clustered index. One or more nonclustered indexes can be created on tables stored as a heap. Data is stored in the heap without specifying an order.
See Microsoft docs on SQL Server.
Oracle: heap-organized table
A heap-organized table is a table with rows stored in no particular order. This is a standard Oracle table; the term "heap" is used to differentiate it from an index-organized table or external table.
See Oracle FAQ's on heap-organized table.
MySQL: memory storage engine
The memory storage engine (formerly known as heap) creates special-purpose tables with contents that are stored in memory. Because the data is vulnerable to crashes, hardware issues, or power outages, only use these tables as temporary work areas or read-only caches for data pulled from other tables.
