It doesn't have a precise definition (which is part of the reason why the people saying programs fundamentally cannot be sentient are so stupid).

But basically it means being conscious / self-aware. Technically it means having some kind of senses that make you aware of the external environment too but that's a minor difference from consciousness - I only said "sentience" because it's what most other people talking about this say. They really mean consciousness. (And also text based IO can be a sense.)