Concepts should be close to objects (physical or logical) and relationships in your domain of interest. These are most likely to be nouns (objects) or verbs (relationships) in sentences that describe your domain.

Noy, Natalya F., and Deborah L. McGuinness. "Ontology development 101: A guide to creating your first ontology." (2001). [-]

A concept is an abstraction; a general idea inferred or derived from specific instances

awson, H., and J.N. Martin. 2008. "On the Use of Concepts and Principles for Improving Systems Engineering Practice". Proceedings of the 18th Annual International Council on Systems Engineering (INCOSE) International Symposium, 5-19 June 2008, Utrecht, The Netherlands. [SEBoK]