Where two planets are applying to aspect but before the aspect perfects another planet overtakes the first and perfects an aspect with the second.
See also: Refranation.
Lilly writes of prohibition (CA., p.110)
Prohibition is when two Planets that signify the effecting or bringing to conclusion any thing demanded, are applying to an Aspect; and before they can come to a true Aspect, another Planet interposes either his body or aspect, so that thereby the matter propounded is hindered and retarded; this is called Prohibition. For example, Mars is in 7. degr. of Aries, and Saturn is in the 12. Mars signifies the effecting my business when he comes to the body of Saturn, who promises the conclusion, the Sun is at the same time in 6. degr. of Aries. Now in regard that the Sun is swifter in motion then Mars, he will overtake Mars, and come to Conjunction with Saturn before Mars, whereby whatever Mars or Saturn did formerly signify, is now prohibited by the Sun his first impediting Mars and then Saturn, before they can come to a true Conjunction. This manner of prohibition is called a Conjunctional or Bodily prohibition; and you must know that the combustion of any Planet is the greatest misfortune that can be.
The second manner of Prohibition is by Aspect, either Sextile, Square, Trine, Opposition, viz. when two Planets are going to Conjunction; as Mars in 7 degree. of Aries, Saturn in 15 of Aries; let us admit the Sun in 5. degr. of Gemini; he then being more swift then Mars in his diurnal motion, doth quickly over-take and pass by the Sextile dexter of Mars ( and comes before Mars can come to Conjunction) to a Sextile dexter of Saturn: This is called Prohibition by Aspect, in the same nature judge if the Aspect be Square, Trine, Opposition.