Meaning

1. middle, center (point roughly equidistant from the edges)
2. middle (point in time that divides something in two roughly equal parts)
3. inside (internal part of something)
4. means (instrument or condition for attaining a purpose)
5. measure (action designed to achieve some purpose)
6. agent (active power or cause or substance; something (e.g. biological, chemical, thermal, etc.) that has the power to produce an effect)