An aggregate of MPDU subframes sharing one preamble.
Reading a capture
The whole point is that the preamble is paid once. Ten frames sent separately at 802.11ax cost ten preambles of 36 microseconds; sent as one A-MPDU they cost one. That is the difference between a link that reaches its rated throughput and one that does not.
Subfields
A-MPDU subframe — One delimiter, one MPDU, and padding to a four-octet boundary.
Further subframes — The remaining subframes, each structured exactly like the one above.