What are orisons? (From Shakespeare's Romeo and Juliet)

Prayers (from the French oraisons).

Juliet: Ay, those attires are best; but, gentle nurse, / I pray thee leave me to myself tonight; / For I have need of many orisons / To move the heavens to smile upon my state, / Which, well thou knowest, is cross and full of sin.

From Shakespeare's Romeo and Juliet; Act IV, Scene 3.