A household or family unit; commonly used in statistics and administrative contexts (しょたい is a more informal variant).
この町の世帯数は年々増えている。
The number of households in this town is increasing year by year.
1世帯あたりの平均収入を調べる。
We examine the average income per household.
世帯主の名前を申請書に書いてください。
Please write the head of household's name on the application form.