Structured Threat Information eXpression. The OASIS standard data format for representing threat intelligence.
STIX 2.1 is a JSON-LD format for representing threat intelligence as a graph of typed objects: attack-patterns (techniques), intrusion-sets (groups), malware, tools, indicators, vulnerabilities, and the relationships between them.
MITRE ATT&CK ships its entire technique catalogue as a STIX bundle, which enables programmatic ingestion. a13e.com pulls technique descriptions, platforms, and data sources straight from the published STIX bundle for every technique page in the library, so the descriptions you read are sourced rather than paraphrased.