There are dozens of definitions of food security in the literature, ranging from the household to the international level. Many of these definitions are narrow, focusing primarily on food supply and the economic viability of various actors in food supply chains. Others are focused on individual responsibility models, essentially denying the role of structural forces in creating food insecurity. Many fail to address issues of race, class, colonialism and justice, and don't specifically address the need for food security initiatives to support household and community empowerment and agency.
The Ryerson Centre for Studies in Food Security provides a range of definitions at the various levels. But I find the most helpful one is their 5As of food security because it can be used at multiple scales, is the most comprehensive and most consistent with the themes of this site.