Final answer:
A dynamic disk is the storage device that allows a partition to be extended into adjacent unallocated space, as long as it is contiguous, offering greater flexibility over basic disks.
Step-by-step explanation:
The term that describes a type of storage device that supports a partition to be extended into adjacent unallocated space, as long as it is contiguous, is a dynamic disk. Unlike a basic disk, a dynamic disk offers the flexibility to create volumes that span multiple disks (spanned and striped volumes), and the ability to extend a simple or spanned volume without the need to restart the system. This is useful when you're running out of space on a volume and need to add more space without data loss, and when the unallocated space is directly next to the partition that you want to extend.