We will test our invention outdoors, unless it rains. What kind of conjunction is the underlined word?

Unless is a subordinate conjunction.

It links the main clause "We will test our invention outdoors" with a subordinate clause "it rains." When looking for a conjunction, many times it's easiest to first look for a comma in the middle of a sentence. A comma in the middle of the sentence often indicates that two clauses are joined together with a comma and conjunction. The independent or main clause can be a sentence all by itself.