Permission to use someone’s data, which has to be informed, specific and freely given to mean anything. Blanket consent buried in terms of service is legally weak in many jurisdictions and ethically weak everywhere. It is also only one of several possible legal grounds, and often not the strongest one.
Technical view
Under regimes such as the General Data Protection Regulation (‘GDPR’), consent is one of six lawful bases and requires an unambiguous affirmative act, granularity per purpose, and equally easy withdrawal. Legitimate interests is frequently a more appropriate basis for analytics, subject to a balancing assessment. For training data, the practical difficulty is that consent obtained for one purpose rarely covers model training, and withdrawal is hard to honour once training has occurred.
Related
Part of AI Terminology Index, section: Data.