Standard weight is a piping designation that denotes the wall thickness. For nominal pipe sizes up to 10", standard weight pipe is equivalent to Schedule 40. Other weight designations are Extra_Heavy or Extra_Extra_Heavy
The term Standard Weight, Extra Heavy and Extra Extra Heavy are from an antiquated sizing system called the iron pipe system (IPS). Schedules are limited to Standard Wall (STD), Extra Strong(XS), and Double Extra Strong (XXS). STD is identical to SCH 40 for NPS 1/8 to NPS 10, inclusive, and indicates 0.375" wall thickness for NPS 12 and larger. XS is identical to SCH 80 for NPS 1/8 to NPS 8 and indicates .500" wall thickness for NPS 8 and larger.