The reaction between an aldehyde ($RCHO$) and hydrazine ($NH_2NH_2$) is a characteristic reaction of the carbonyl group ($>C=O$).

1. Nucleophilic Addition: The nitrogen atom of hydrazine acts as a nucleophile (due to its lone pair of electrons) and attacks the electrophilic carbon of the carbonyl group. This is consistent with the behavior of carbonyl compounds described in organic chemistry principles [1], [2].
2. Elimination: The intermediate addition product undergoes the elimination of a water molecule ($H_2O$) to form the final product, a hydrazone ($RCH=N-NH_2$).

Therefore, the overall mechanism is classified as Nucleophilic addition – elimination.