Consider the reaction

$\mathrm{R}-\mathrm{CHO}+{\mathrm{H}}_2\mathrm{N}-\mathrm{OH}\to \mathrm{R}-\mathrm{CH}=\mathrm{N}-\mathrm{OH}$

What type of reaction is this ?

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