GIVT is General Invalid Traffic, and SIVT stands for Sophisticated Invalid Traffic. The MRC (Media Ratings Council) has devised a definition for it. It avoids using fraud in blanket terms and hence gives an "invalid" traffic classification.
GIVT, as per MRC, consists of traffic identified through routine means of filtration executed through the application of lists or with other standardized parameter checks. Few examples of GIVT traffic include "Data Center Traffic," "Bots, Spiders and Crawlers," "Activity-based filtration," "Non-browser UA Headers or other unknown browsers," "pre-fetch or browser pre-rendered traffic."
SIVT, as per MRC, consists of more difficult to detect situations that require advanced analytics, multi-point corroboration/coordination, significant human intervention, etc., to analyze and identify. Few examples are "Differentiating human and IVT if they are originating from the same source," "Bots masquerading as legit users," "Hijacked user sessions, devices, etc.," "Non-viewable / obfuscated ad serving," "Invalid Proxy Traffic," "Adware and Malware," "Incentivized traffic," "Falsified viewable impressions," "Falsely represented sites," "Cookie stuffing, harvesting and recycling," "Falsification of location data."
You can also download the MRC Invalid traffic guidelines from here.