These words apply to things that are not marked by change or variation. Something is invariable if it recurs always in the same manner or at regular intervals. The succession of neap tide and spring tide is invariable . Something is constant if it remains the same or changes at a regular rate over a long period of time. The birth rate in some countries is constant , and in other countries the rate of population increase is constant . In mathematics and science, a constant is a number or quantity that does not change throughout a given discussion or operation. An absolute constant is fixed and cannot be changed. An arbitrary constant can be assigned any set value. In a more general sense, invariable and constant may simply mean continual or habitual.

• She invariably forgets to take her keys; She is constantly changing her mind.

And while invariable more often applies to routine, both words may be used of abstract qualities that are enduring or reliable: an invariable stop at the corner news-stand; constant affection; invariable amiability.

Something is unchanging in this sense if its attributes remain the same for a long period of time. Weather and climate may be said to be unchanging in certain areas of the world. The biological processes of life may be called unchanging .

In one sense, uniform is close to constant, describing something that does not undergo changes in form or character, quantity or degree: a thermostat to maintain a uniform temperature. In another sense, uniform may suggest a bringing of separate things into a state of conformity, either basically or in some salient aspect: progress towards the adoption of a uniform building code; The hedges were clipped to a uniform height; all twenty volumes of the encyclopaedia were of uniform thickness.