A seniūnija (in English: eldership, elderate, ward, parish, or subdistrict) is the smallest administrative division of Lithuania. An eldership could either be a very small region consisting of few villages, one single town, or part of a big city. Elderships vary in size and population depending on their place and nature. A few elderships make up a municipality. Šilainiai (Kaunas) and Dainava (Kaunas) are the most populous elderates, with population counts over 70,000, exceeding the population of some entire municipalities.
Elderships manage small scale local matters, such as repairing pavements, dirt roads, keeping records on all families living in the eldership. Its premise is that unlike with higher administrative divisions, an elder (leader of eldership) could have time to talk to every person in the eldership who wants to.
Modern Lithuania is divided into 10 counties, 60 municipalities, and 546 elderships. Elderships function as municipal districts. In cities, an elder is elected to each eldership district, along with a mayor for the city.
Note, historically the term was used for significantly bigger administrative units, e.g. Eldership of Samogitia (also known for a longer period of history as Duchy of Samogitia).
More information on the Wikipedia page