General synonyms:
* Connector
* Linker
* Joiner
* Combiner
More specific synonyms:
* Coordinate: For conjunctions that join elements of equal grammatical rank (e.g., "and", "but", "or")
* Subordinate: For conjunctions that join elements of unequal grammatical rank (e.g., "because", "although", "since")
* Transition: For conjunctions that connect sentences or paragraphs (e.g., "therefore", "however", "moreover")
* Adverbial conjunction: For conjunctions that function like adverbs (e.g., "therefore", "however", "consequently")
The best synonym for conjunction will depend on the specific context in which you are using it.