Reaction between benzaldehyde and acetophenone in presence of dilute NaOH is known as

Cross aldol condensation reaction:  

A few steps take place in this reaction

Step 1: Acid-base reaction occurs between base and alpha hydrogen of acetophenone, and carbanion is formed.

Step 2: In this step, formed carbanion behaves as a nucleophile and attacks carbonyl compound (benzaldehyde) carbon to form beta-hydroxy carbonyl compound.

Step 3: $\mathrm{\alpha },\mathrm{\beta }$-unsaturated carbonyl compound is formed as a product after heating.