Name the phase of viral replication where the virus enters the host cell.

The phase of viral replication during which the virus enters the host cell is referred to as penetration. This stage follows the initial attachment of the virus to the host cell's surface. During penetration, the viral particle or its genetic material is introduced into the cell’s interior, enabling it to initiate the process of replication and subsequently take over the host's cellular machinery.

This phase is critical because it sets the foundation for the subsequent steps of the viral lifecycle, which include replication and assembly of new viral particles. Proper understanding of this phase is crucial for grasping how viruses hijack host cells to propagate, which is an important concept in both microbiology and virology.