Definitions of yet:

• adverb:   up to the present time

  Example: "I have yet to see the results"

• adverb:   to a greater degree or extent; used with comparisons

  Example: "A yet sadder tale"

• adverb:   used after a superlative

  Example: "The largest drug bust yet"

• adverb:   within an indefinite time or at an unspecified future time

  Example: "He longed for the flowers that were yet to show themselves"

• adverb:   despite anything to the contrary (usually following a concession)

  Example: "He was a stern yet fair master"

• adverb:   used in negative statement to describe a situation that has existed up to this point or up to the present time

  Example: "The sun isn't up yet"