The concept and definition of Antenna Ports are same as LTE Antenna Ports. It is defined as follows (38.211-4.4.1).

An antenna port is defined such that the channel over which a symbol on the antenna port is conveyed can be inferred from the channel over which another symbol on the same antenna port is conveyed.

What does this mean ? I interpret this as follows :

Each Antenna ports carries its own resource grid and a specific set of reference signal in the grid. The channel properties for RE(resource element) for the reference signal is assumed to be same (or very close to same) as the resource elements for other data(e.g, REs for PDSCH). Due to this facts, we can help demodulate the data by using the channel information obtained by the anaysis of reference channel.

In NR, a certain range of antenna port number is assigned for each channel and signal as follows.

<38.211 - 6.2, 7.2>

A few notes from 38.211 - 6.2 are :

• When using PUSCH repetition Type B, the PUSCH symbols on the same uplink antenna port can infer each other if the two symbols correspond to the same actual repetition of a PUSCH transmission with repetition Type B.
• If there's no intra-slot frequency hopping and PUSCH repetition Type B isn't applied, symbols on the uplink antenna port can infer each other if the two symbols correspond to the same slot.
• With intra-slot frequency hopping enabled, symbols on the uplink antenna port can infer each other only only if the two symbols correspond to the same frequency hop, regardless of whether the frequency hop distance is zero or not.
• For DM-RS bundling on PUSCH and/or PUCCH repetition and/or multi-slot transport-block processing, the symbols on the uplink antenna port can infer each other if the two symbols are transmitted within the same actual time-domain window.

A few notes from 38.211 - 7.2 are :

• Unless otherwise specified, the UE shouldn't assume that two antenna ports are quasi co-located with respect to any QCL type unless specified
• For DM-RS linked to PDSCH: A PDSCH symbol on an antenna port can infer information from a DM-RS symbol on the same port if both symbols fall within the same scheduled PDSCH resource, are in the same slot, and belong to the same PRG
• For DM-RS linked to PDCCH: A PDCCH symbol on an antenna port can gather details from a DM-RS symbol on the same port only when both symbols are in resources where the UE presumes identical precoding is used
• For DM-RS associated with PBCH: A PBCH symbol on one antenna can deduce data from a DM-RS symbol on the same antenna if both symbols belong to a SS/PBCH block sent within the same slot and share the same block index

Reference

[1]