What is Tikanga?
Tikanga is the broader term that includes Kotahitanga.
What does Tikanga mean?
noun:   customs and traditional values, especially in a Maori context. In a sentence: "Maori have a right to do things their way, in accordance with their tikanga"
Kotahitanga is one of those values that is included in Tikanga.
Some of the other values that are included in Tikanga are Whanaungatanga (A sense of belonging), Manaakitanga (It is a measurement of people's ability to extend aroha) and Maramatanga (understanding). There are many more aspects of Tikanga that aren't here, you can find them in the link below, along with a lot of other useful information about both Kotahitanga and Tikanga.
Tikanga’s values     What is kotahitanga?     Tikanga’s values How Tikanga is relevant