Synonyms for Concentration (same or very similar meaning)
WordNet sense 1 (complete attention; intense mental effort):
absorption, engrossment, immersionWordNet sense 2 (the spatial property of being crowded together):
compactness, denseness, density, tightnessWordNet sense 3 (great and constant diligence and attention):
assiduity, assiduousnessFrom the ODE community, based on WordNetadd/edit