What is the Specific Type of Word that Includes Stellar, Sylvan, etc

The process you seem to be concerned with is derivation:

the process of forming a new word on the basis of an existing word, e.g. happiness and unhappy from happy, or determination from determine. It often involves the addition of a morpheme in the form of an affix, such as -ness, un- and -ation in the preceding examples.

As John Lawler explains, sylvan and stellar are derived from the Latin roots stella and sylva through addition of the Latin affixes -ar and -an. Some English affixes used to derive adjectives from English roots (and sometimes roots from other tongues) are -y, -ish, -like. For more examples you may Google "derived adjectives" or --as you have now pointed out to me-- "deonominal adjectives" ('un-nouned').