Characteristic - श्लेषः स वाक्ये एकस्मिन् यत्रानेकार्थता भवेत् ।
It means that where there can be more than one meaning of a sentence that figure of speech is Pun.
For example - केशवं पतितं दृष्ट्वा पाण्डवा हर्षमागताः । रूदन्ति कौरवाः सर्वे हा हा केशव केशव ॥
(Meaning of Shloka: 1. Seeing Keshva i.e. Krishna fell the Pandavas (including Yudhishthira) rejoiced. All the Kauravas (including Duryodhana) started crying saying oh; Krishna! oh; Krishna !)
(The words केशवः, पाण्डवाः and कौरवाः have two meanings. We find one meaning in the above noted translation. The other meaning of the shloka with words के means that which has fallen in water, शवम means a dead body. पाण्डवाः means fish and कौरवाः means crows. Seeing a dead body in water, fish rejoiced and crows cawed.)
There are two meaning of the words केशवम् - Krishna and a dead body in water, पाण्डवाः-Pandavas and fish, कौरवाः Kauravas and crows. The two meaning of the words create wonder here. There is 'Pun' figure of speech here as the words have many meanings.
(कौ कौ)