Encroachment is specifically the act of:

Encroachment refers to the act of intruding on another person's property, typically by having a physical structure or improvement extend over the property line. This can happen when a fence, building, or tree branch from one property crosses into an adjacent property without the owner’s permission. Encroachment can lead to legal disputes between property owners regarding property rights, as it involves a violation of boundaries established by property lines. In real estate law, recognizing and addressing encroachments is crucial to maintain clear ownership and avoid potential conflicts.

The other options presented relate to different concepts that don't align with the definition of encroachment. Thermal expansion pertains to changes in material size due to temperature variations, which has no bearing on property boundaries. An improvement on public land could involve developments that enhance communal resources but do not signify an intrusion onto private property. Land acquisition by the government typically deals with eminent domain and property claim processes rather than unauthorized physical intrusion. Hence, the correct understanding of encroachment is directly tied to its definition as an intrusion on another’s property.