Nitya karma refers to those karmas (or rituals) which have to be performed daily by Hindus. The Hindu Shastras say that not performing nitya karmas leads to sin. The nitya karmas include:
Nitya Karma does not necessarily mean daily duties. It includes any regular/periodic scheduled activities/duties. E.g.: Amavasya tharpanam, Grahana tharpanam, Pithru devasam.
There is a subdivision of Nithya karma which is called Nai-Nithya karma. This means compulsory karma but conditional. E.g. Grahana related karmas
See also