Final answer:
A phage that lacks critical DNA for replication is called defective, meaning it cannot produce new virions and complete its replication cycle.
Step-by-step explanation:
If the infecting phage lacks some critical pieces of DNA necessary for replication, it is called defective. This occurs when the phage cannot complete its replication cycle due to missing genetic information needed to produce new virions. Bacteriophages require specific genes to bind to host cells and insert their genome, which are then used by host enzymes to replicate and transcribe the viral DNA. If a phage cannot direct the biosynthesis of new viral components due to a lack of critical genes, it does not engage in a productive infection and is considered defective.