In
UK regional usage, "semi" is short for "semi-detached house", that is one of a pair of houses that form a single building but provide two residences. Imagine a single,
detached house split vertically in half. The two sides form a mirror-image pair of semis. Generally found alongside other similar houses in a street. However, in rural areas, may be found in splendid isolation.
In broad terms, a semi is more expensive than a terraced, cheaper than a detached.