Polyandry is a marriage arrangement in which one woman is married to multiple men at the same time.
Normally, in Mormon-usage, polygamy does not encompass polyandry, although, by definition, polyandry is a subset of polygamy.
LDS doctrine does not apparently permit polyandry. Nevertheless, there exists some debate regarding supposed polyandrous marriages Joseph Smith may have been involved in. The exact nature of these relationships is not well known enough to really be able to determine if there was any real polyandry. It may also be that there is more to correct doctrine than is superficially apparent.