Myanmar is India's only land neighbour inside ASEAN, with a 1,643-km land border
Background
The Embassy of India in Yangon describes India and Myanmar as immediate neighbours linked by a 1,643-km land border and a maritime boundary of 1098 kms in the Bay of Bengal. The embassy calls Myanmar the only ASEAN country sharing both a land and maritime boundary with India, and a gateway connecting India's Northeast with Southeast Asia. The embassy records cooperation spanning trade, connectivity, border management and, among other fields, development partnership, capacity building, energy, health, education and cultural exchanges. The embassy says the relationship is guided by India's Neighborhood First, Act East, Indo-Pacific and MAHASAGAR visions, and that the two countries share deep-rooted historical, civilizational, cultural and people-to-people ties.
In plain words
ASEAN Association of Southeast Asian Nations — A ten-plus-member regional grouping of Southeast Asian states founded by the Bangkok Declaration.
Act East Act East Policy — India's policy of deepening economic and strategic links with Southeast and East Asia.