"Significant heterogeneity" means a noticeable or important amount of diversity or variability within a particular group, situation, or data set. Full definition
Findings were characterized by significant heterogeneity across studies that was partially explained by the results of moderator analyses. (link.springer.com)
Although the results of the meta - regression showed no evidence of significant heterogeneity between subgroups, summary association estimates were slightly different in subgroup analyses by study design and exposure assessment. (academic.oup.com)
Authors found significant heterogeneity in treatment effects in the studies, so results should be treated with caution. (ebmh.bmj.com)