Def: NOSIG is one (unique) type of TREND. Effectively meaning, the forecast for 2 hours is "weather unchanged" from the METAR/SPECI observation.
WMO Manual on Codes vol I.
http://library.wmo.int/opac/index.ph...splay&id=13617
15.14.15 When none of the elements listed in Regulation 15.14.2 is expected to change significantly as to require a change to be indicated, this shall be indicated by the code
word NOSIG. NOSIG (no significant change) shall be used to indicate meteorological conditions which do not reach or pass specified threshold criteria.
Other references reference in ICAO ANNEX 3 too
e.g. here.
6.3 Landing forecasts
6.3.1 A landing forecast shall be prepared by the meteorological office designated by the meteorological authority concerned as determined by regional air navigation agreement; such forecasts are intended to meet the requirements of local users and of aircraft within about one hour’s flying time from the aerodrome.
6.3.2 Landing forecasts shall be prepared in the form of a trend forecast.
6.3.3 A trend forecast shall consist of a concise statement of the expected significant changes in the meteorological conditions at that aerodrome to be appended to a local routine or local special report, or a METAR or SPECI. The period of validity of a trend forecast shall be 2 hours from the time of the report which forms part of the landing forecast.