The environment inside the cell when water moves in and it swells is called hypotonic.

Here's why:

* Hypotonic means the concentration of solutes (dissolved substances) is lower outside the cell than inside.

* This difference in concentration creates a concentration gradient, causing water to move across the cell membrane from the area of higher concentration (outside) to the area of lower concentration (inside) to try and equalize the solute concentrations.

* This movement of water into the cell causes it to swell.

Let me know if you'd like to explore other terms related to cell environments!