What is Days On Market?

Legal Definition
Days on market (DOM, alternatively active days on market, market time, or time on market) is a measurement of the age of a real estate listing. The statistic is defined as the total number of days the listing is on the active market before either an offer is accepted or the agreement between real estate broker and seller ends.

Generally, properties with a large DOM value will command lower prices than properties with few DOM because a perception exists that the property may be overpriced or less desirable. DOM often factors into developing a pricing strategy. DOM can also be used as a "thermometer" to gauge the temperature of a housing market.

This value is not necessarily how long the house has been on the market due to intricacies within the multiple listing service (MLS) database. Depending on the rules of the MLS that is being used, the number is reset if a seller switches real estate agents. Sometimes there is also the arguably unethical practice of "withdrawing" the listing before it expires and adding the listing again to reset the DOM. As a result, when this statistic is used it is often lower than the true value. However, savvy real estate agents (if the MLS allows) will research the property's listing history and can tell more effectively how long the property has been on the market.
-- Wikipedia