Silent Assassin is, as with the previous game, the highest rating you can achieve within a mission. The requirements to achieve this rating, differ slightly from the previous game, but the principle is the same, be the perfect assassin.
To achieve the silent assassin rating, you need to complete your mission without having your cover blown and without there being any witnesses to your actions. Also, you cannot be caught on a security camera, unless you manage to acquire the tape that you have been recorded on. No-one can be injured or killed except your target, unless it was an 'accident'.
On expert and professional difficulty, all of the previous stipulations must be followed, but there are more with it being a harder difficulty. On expert and professional, you also cannot leave any evidence behind, which is basically your suit if you change disguise and any custom weapons that you take with you must also be retrieved. Also, no bodies may be found, so anyone you kill, which should only be your target, must be hidden unless the kill was staged to look like an accident.
These ''5 Silent Assassin ratings'' achievements are stackable, so getting 5 Silent Assassin ratings on professional will unlock the lower difficulty achievements, too.
You can get a silent assassin rating on the same mission, 5 times and you will still get the achievement.