Consider the reaction $\mathrm{RCHO}+{\mathrm{NH}}_2{\mathrm{NH}}_2\to \mathrm{RCH}=\mathrm{N}-{\mathrm{NH}}_2$ What sort of reaction is it?

The reaction RCHO + NH₂​NH₂ ​→ RCH=N−NH₂​ is a nucleophilic addition-elimination
reaction. First, a molecule of hydrazine is added to the carbonyl group. This is a nucleophilic addition reaction.
RCHO + NH₂​NH₂ ​→ RCH(OH)−NH−NH₂​
Then, a molecule of water is eliminated. This is an elimination reaction.
RCH(OH)−NH−NH₂ ​→ RCH=N−NH₂ ​+ H₂​O