A single unit of
attention. Each person is born with an
arbitrary number of attention units.
This is part of the Hubbard model of the human mind. Attention units are part of the analyzer. They can be partitioned off into segments and perform tasks apart from the main consciousness of the individual - for example, listening to music at the same time as driving. When the task is done, they are released and are again available for other uses.
Attention units can also be partitioned by engrams, which is bad because engrams usually aren't quick to release attention units.
See also Self Analysis and Dianetics.