* Abbreviation: It might stand for something, like "transaction" or "translation". In this case, its part of speech would depend on the full meaning of the abbreviation.
* Typo: Perhaps you meant a different word entirely.
* Part of a larger word: It might be a syllable within a word, like "transport" or "triangle".
To figure out the part of speech, you need to provide more context. Please tell me where you encountered "tr" and what the surrounding words are.