Here's a METAR:
SCEL 090000Z 15005KT 5000 HZ NSC 10/08 Q1020 NOSIG
What does NSC mean?
WMO Publication No. 306 - Manual On Codes - Volume I.1 - Part A, Section A, FM 15 METAR, FM 16 SPECI:
15.14.14 When no cloud below 1 500 metres (5 000 ft) or the highest minimum sector altitude, whichever is greater, and no cumulonimbus and no towering cumulus are forecast, and CAVOK is not appropriate, the abbreviation NSC shall be used.
Some countries have published different rules:
220.127.116.11 Abbreviation NSC: The abbreviation NSC shall not be used. As appropriate, clouds present at and above 5 000 ft also shall be reported.
18.104.22.168 Other than at Auckland (NZAA), Wellington (NZWN) and Christchurch (NZCH) NSC is not used, clouds are reported at all heights and SKC is used to report no cloud.
AIP USA, GEN 1.7 - Differences From ICAO Standards, Recommended Practices and Procedures, Annex 3 − Meteorological Service For International Air Navigation, Part II, Appendix III:
4.5.4 ... CAVOK and NSC are not used.
And the Federal Meteorological Handbook No. 1 also does not have NSC in the METAR definition.