Definitions

1. interjection

a greeting or a signal to begin or continue communication, especially in radio or telephone communication

“I called out ‘allo’ to signal that I was ready to begin the conversation.”

2. noun

a greeting or a call to attract attention

“The sound of ‘allo’ echoed through the empty streets.”

3. verb

to make a noise to attract attention or to signal the start of communication

“The bird began to allo, signaling the start of the dawn chorus.”