SLP

• sea level pressure

  METAR

• slope

  General

• speed limiting point

  ICAO

SLP has 3 meanings; the right one depends on context. A weather report, a NOTAM or an ATC clearance. Got a NOTAM full of contractions? Paste it into the decoder.

Source: FAA Order JO 7340.2 Contractions. Not for operational use.