Some common synonyms of apposite are applicable, apropos, germane, material, pertinent, and relevant. While all these words mean "relating to or bearing upon the matter in hand," apposite suggests a felicitous relevance.

// add an apposite quotation to the definition

The words relevant and apposite can be used in similar contexts, but relevant implies a traceable, significant, logical connection.

// found material relevant to her case

In some situations, the words apropos and apposite are roughly equivalent. However, apropos suggests being both relevant and opportune.

// the quip was apropos