"Adhik Masa" is a word of Sanskrit origin which literally means Extra month. Adhik Masa is also known as Purushottam Masa, Mal Masa, and Malimmacha etc. Adhik Masa has started today i.e. April 15, 2010 and will end on May 14, 2010 as per English calendar. Because the extra month this year is Vaishakh as per Hindu Calendar, this extra month is known as Adhik Vaishakh Masa or Adhik Vaishak Mahina. So in current year there will be thirteen months in Hindu calendar. Apart from the normal 12 months, there is an additional month i.e. Adhik Vaishakh Masa.
Acharya Ram Hari Sharma has written a detailed article on "Rituals & Significance of Adhik Maas" on AstroCAMP.com Astrology Magazine
“Generally all efforts with the intention of worldly gains are strictly prohibited during Adhika masa as mentioned in scripture and one is advised to follow special rituals for this month. During this period, marriages, tonsures (mundan), inaugurations of new home, fasting with desire of some worldy gains, buying new ornaments and vehicles are prohibited.
The month that does not carry Surya Sankranti is called Adhimass and the month carrying two Sankrantis is called Kshayamass. As per Astrological calculations, Adhimass comes after 32 months, 16 days and 4 ghatis and Kshayamaas first arrives after 141 years and then after 19 years.”