'Long-term' refers to more that 100 years.
Ecoinvent decribes it as follows in its Overview and Methodology:
Emissions that occur over large time frames of substantially more than 100 years are assigned to specific subcategories (labelled ‘long term’). Such long-term emissions occur in landfill sites (leaching), in uranium mining and milling sites (radon emissions) and – probably – final repositories of nuclear waste. The ecoinvent database contains corresponding long-term subcategories in order to distinguish these long-term emissions from the ones occurring within the first 100 years.
For further explanation on this long-term emission topic, please refer to this ecoinvent page.