What is adhika?

Some articles on adhika:

Hindu Calendar - Months of The Lunisolar Calendar - Religious Observances in Case of Extra and Lost Months
... Among normal months, adhika months, and kshaya months, the earlier are considered "better" for religious purposes ... are two Āśvayuja (Āśvina)' months caused by the existence of an adhika Āśvayuja, the first adhika month will not see the festival, and the ... is āshvayuja kshaya then the festival will be observed in the first adhika month itself ...
Hindu Calendar - Months of The Lunisolar Calendar - Month Names - Lost Months (Kṣaya Māsa)
... February 13 onwards was (Adhika) Phālguna ... the second month and take the epithet Adhika and the next month will be labelled by both its transits as is usual for a Kṣaya-Māsa This is a very very rare occurrence ... October 8 to November 5 were Kārtika Adhika-Māsa ...
Hindu Calendar - Another Kind of Lunisolar Calendar
... months are defined by the full moon, the adhika and kṣaya lunar months are still defined by the new moon ... is, even if the pūrnimānta system is followed, adhika or kṣaya months will start with the first sunrise after the new moon, and end with the new moon ... The adhika month will therefore get sandwiched between the 2pakṣa-s of the nija months ...