Lot

A lot is a part of a neighborhood where build mode and buy mode objects can exist.

The Sims
In The Sims, Sims could not go outside lots.

Lots are mainly divided into residential lots where Sims live, and community lots where they can interact with other Sims.

The Sims 2
Like in The Sims, Sims cannot go outside of lots in The Sims 2

The Sims 2 expansion packs added new lot types - such as dorms and secret societies (University), hotels and beaches (Bon Voyage) and apartments (Apartment Life).

The Sims 3
The Sims 3 allows Sims to explore lands between lots. Sims cannot route over water, objects, and terrain specified as non-routable in the Create a World world-building tool.

The Sims 3 introduced a much broader range of lot assignments such as library and gym, alongside rabbit hole objects that affect lots. These assignments and objects can change a lot's properties.