【Inconstant】 , 【fickle】 ,  【capricious】 ,  【mercurial】 ,  【unstable】  mean lacking or showing lack of firmness or steadiness in purpose, attachment, or devotion.

【Inconstant】 , usually applied to persons though sometimes to things, suggests an inherent or constitutional tendency to change frequently; it commonly implies an incapacity for fixity or steadiness (as in one’s affections, aspirations, or course).

【Fickle】 retains only a hint of its basic implication of deceitfulness or treacherousness, but its basic implications of instability and unreliability are colored by the suggestion of an incapacity for being true, steadfast, or certain.

【Capricious】 suggests qualities which manifest or seem to manifest a lack of guidance by a power (as law, authority, or reason) that tends to regularize movements or acts. When used in reference to persons, it suggests guidance by whim, mood, freak, or sudden impulse.

When used in reference to things, it implies an irregularity, an uncertainty, or a variableness that seems incompatible with the operation of law.

【Mercurial】 is a synonym of the other words here discriminated only when it carries a strong implication of resemblance to the metal mercury and its fluctuations when subjected to an external influence. The word, however, also carries implications (as of swiftness, eloquence, cleverness, and volatility) derived from its earlier association with the god Mercury.

Consequently when it applies to persons, their temperaments, or their natures, it usually suggests a pleasing even if baffling variability, an amazing succession of gifts capable of being displayed at will or at need, and such other qualities as sprightliness, restlessness, flashing wit, and elusive charm.

【Unstable】 , which is applicable to persons as well as to things, implies a constitutional incapacity for remaining in a fixed position mentally or emotionally as well as physically; it suggests, therefore, such fluctuations in behavior as frequent and often unjustified changes in occupation or in residence or sudden and startling changes of faith or of interests.