H. P. Blavatsky

Sandhyâ. A period between two Yugas, morning‐ evening; anything coming between and joining two others. Lit., “twilight”; the period between a full Manvantara, or a “Day ”, and a full Pralaya or a “Night of Brahmâ”.

Source: H. P. Blavatsky, The Theosophical Glossary, art. Sandhya