In reference to the Buddhist canon (the "suttapitaka" especially) the meaning of the term is equivalent to the English "Volume" or "Tome" --although the works in question are so voluminous that few (unexpurgated) editions can fit an entire Nikaya into a single volume each.
The word has other usages, we should note particularly that in Theravada Buddhism of south-east Asia it is the polite term for monastic orders, divisions among monks, or "sects".
The Chinese equivalent is "Agama", and the Suttapitaka of the Buddhist canon is sometimes referred to as "The Agamas" by Mahayana Buddhists --or as "The Nikayas" if they are in the company of Theravada Buddhists.