Maksotov post #4 has the correct definition. Some of the other posts have it backwards. I nearly gave up trying to confirm that transits occur in inferior conjuctions. The following is partial quote from Transits of Venus at www.wickipedia.org Normally when the Earth and Venus are in conjunction they are not aligned with the Sun. Venus' orbit is inclined by 3.4° to the Earth's so it appears to pass under (or over) the Sun in the sky.[3] Transits occur when the two planets happen to be in conjunction at (or very near) the line where their orbital planes cross. Although the inclination is only 3.4°, Venus can be as far as 9.6° from the Sun when viewed from the Earth at inferior conjunction.[4] Since the angular diameter of the Sun is about half a degree, Venus may appear to pass above or below the Sun by more than 18 solar diameters during an ordinary conjunction.[3]