What is the name of the mode with infinite waves of zombies?

The mode with infinite waves of zombies is Survival: Endless.

Survival: Endless is the final and most open-ended Survival mode in the original Plants vs. Zombies. Instead of ending after a fixed number of flags, it continues through increasingly difficult waves, allowing players to test how long a carefully designed defense can last. The mode takes place on the Pool or Roof, depending on the selected version and stage setup.

The mode is often confused with Survival: Hard, but Hard describes the difficulty of a stage with a limited sequence of waves. Survival: Endless is defined by its continuing progression. Its escalating difficulty comes from tougher zombie combinations, more frequent special zombies, and increasingly demanding resource management. Players commonly build elaborate, repeatable defenses using plants such as Cob Cannon, Winter Melon, Gloom-shroom, and Pumpkin. Because the game can eventually produce enormous mixed waves, the challenge is less about simply completing a level and more about maintaining a stable strategy indefinitely.