A segment is a logical storage structure that contains data structure (segment type) of database objects.
For example:
One segment name is one object but one object may have several segments records.
A segment is uniquely defined with the following columns:
A segment is an set (unordered) of extents. A segment and all its extents are stored in one tablespace.
Oracle Database allocates space for segments in units of one extent. When the existing extents of a segment are full, Oracle Database allocates another extent for that segment. Because extents are allocated as needed, the extents of a segment may or may not be contiguous on disk.
Within a tablespace, a segment can include extents from more than one file; that is, the segment can span datafiles.
Data Structure (Segment Type)
As a segment is an object, you must refer to the object (alter, create,…) statement to modify them.
Example: if you want to suppress a lobindex segment, you need to suppress the lob column.