The phrase "a propos" is often used to introduce a relevant or related topic in conversation. However, there are various synonyms that one can use to convey the same idea. One such synonym is "relevant," which means that the topic being discussed is closely connected to the matter at hand. Another synonym is "pertinent," which refers to the relevance of something to a particular situation or circumstance. "Applicable" is another synonym, suggesting that an idea or topic is suitable and appropriate to the current circumstances. Finally, "appropriate" is a more general synonym, indicating that the topic being introduced is fitting or suitable for the context in which it is being discussed.