1. Logical Volume Manager: Most Unix-like systems now have the ability to break up physical discs into some number of units. Storage units from multiple drives can be pooled into a "logical volume", where they can be allocated to partitions. Additionally, units can be added or removed from partitions as space requirements change. From Linux Guide @FirstLinux

2. LVM includes all of the support for handling read/write operations on physical volumes (hard disks, RAID-Systems, magneto optical, etc., multiple devices (MD), see mdadd(8) or even loop devices, seelosetup(8)), creating volume groups (kind of virtual disks) from one or more physical volumes and creating one or more logical volumes (kind of logical partitions) in volume groups. From Redhat 8.0 RPM