Human adaptation is a fundamental process that shapes how individuals respond to cognitive, emotional, behavioral, and physical challenges. Despite its centrality to human functioning, adaptation-related knowledge remains fragmented across affective computing, neuroergonomics, digital phenotyping, rehabilitation engineering, and human digital twins. These fields have advanced the assessment, monitoring, and prediction of human conditions, yet adaptation is frequently treated as a secondary outcome rather than the primary phenomenon of interest. This paper introduces Human Adaptation Engineering (HAE), a conceptual framework that positions adaptation as the integrative construct linking diverse human-centered research areas. A literature-based conceptual synthesis was conducted using fifty highly cited review, survey, and framework papers published between 2015 and 2026. Qualitative analysis identified recurring adaptation processes—disruption, response, adjustment, stabilization, and long-term outcome—across cognitive, emotional, behavioral, and motor dimensions. The HAE lifecycle model and five conceptual propositions provide a domain-agnostic scaffold for theory development, interdisciplinary collaboration, and adaptation-centered system design. The paper discusses theoretical and practical implications, acknowledges limitations of citation-based synthesis, and outlines a future research agenda for operationalizing and empirically validating adaptation as a cross-domain engineering target.