1. likely to change suddenly or unexpectedly; not steady or secure
2. not firmly fixed; likely to move, fall, or collapse
3. having unpredictable emotions or behavior; mentally or emotionally unbalanced

Examples

• The political situation in the region remains unstable.
• Don’t sit on that unstable chair—it might break.
• The ladder felt unstable against the wall.
• His unstable mood swings worried his family.
• The unstable compound exploded when heated.
• Economic conditions have been unstable for months.
• She struggled with an unstable relationship with her parents.