What is the legal term for entering an unoccupied structure without permission?

The correct answer is burglary. Burglary is specifically defined as the unlawful entry into a building or structure with the intent to commit an illegal act, typically theft. This legal term encompasses not just the act of entering, but also the intention behind the entry, which distinguishes it from other related terms.

In contrast, while breaking and entering and trespassing both involve entering property without permission, they do not necessarily imply the intent to commit a crime inside. Breaking and entering can refer to the act of forcibly entering a structure, but it is often used in conjunction with burglary to describe the unlawful entry that intends to commit a crime. Trespassing generally refers to entering someone's property without permission but does not include the element of intent to commit an illegal act within the structure. Criminal entrance is not a widely recognized legal term.

Thus, the concept of burglary captures both the unlawful entry and the intent to engage in further criminal activity once inside, which makes it the most accurate term in this context.