To say Aunt in Thai you have to specify whether it’s your father’s or mother’s older or younger sister.

Aunt from Father’s side

If it’s your father’s younger sister then aunt is อา (aa).

If it’s your father’s older sister then aunt in Thai is ป้า (bpâa).

Aunt from Mother’s side

If it’s your mother’s younger sister then aunt is น้า (náa).

If it’s your mother’s older sister then aunt in Thai is ป้า (bpâa).

Learn the words for uncle here

Example

• Today is relatives gathering day. There is everyone (father, mother, uncles, aunts).
  วันนี้เป็นวันรวมญาติ · มีทั้งพ่อแม่ลุงป้าน้าอามาครบทุกคน
  wan née bpen wan ruam-yâat · mee táng pôr mâe lung bpâa náa aa maa króp túk kon

Summary

To summarize, aunt in Thai depends on whether it’s your aunt from the father’s or mother’s side and also whether it’s their older or younger sister.

In both sides, the older sister is ป้า (bpâa).

In your father’s side if its the younger sister then aunt in Thai is อา (aa).

In your mother’s side if its the younger sister then aunt is น้า (náa).