These words refer to what is substandard or low in quality. Deficient is by far the most specific and exact in meaning. The word points to a lack of something that is required to accomplish a given goal or purpose; a cheque returned because of deficient funds; Rickets can result from a diet deficient in vitamin D. Usually the word does not suggest a lack that is a matter of degree but one, whether small or large, that fails to meet a minimum level of need below which something cannot function at all. Applied to aspects of character, the word suggests a flaw or failing, whether correctable or not, that makes someone unable to perform some activity: an armchair theorist deficient in practical experience.

Poor , the least specific and most wide-ranging of all these words, applies in this context to a matter of degree rather than to a cut-off point at which something ceases to function: poor eating habits that, miraculously, did not result in a vitamin deficiency; anaemia that resulted from a diet poor in iron. Inadequate relates more closely to deficient in indicating a failure to meet minimal standards. Deficient , however, often points to a minimum that can be objectively measured whereas inadequate can point to a more subjective evaluation: deficient by two votes of passing the proposed legislation; a pianist whose technique seemed inadequate to the demands of the pieces he had chosen to play. But inadequate can indicate lacks that are a matter of fact rather than of taste: a water supply inadequate to the needs of the city. By contrast, unsatisfactory stresses the first sense of inadequate , pointing more to subjective evaluation than to measurable lack: The teacher informed him that he was making unsatisfactory progress in his studies. It can also indicate a matter of degree, like poor : many brilliant scenes in a ply that on the whole was unsatisfactory . As in the last example, the word at its most literal can refer to a failure to give satisfaction or pleasure.