Seeing the tiger, the man ran away.
I'd like to know whether 'seeing' is gerund or participle? may be explained.
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It is a participle. A participle functions (externally) like an adjective; the word seeing describes and modifies the man in your example, so it is a participle.
Separating the participle from the noun it modifies by a comma, as in your example, is called apposition.
A gerund functions like a noun; if it were a gerund, then it would be hanging in the air in your example. You can test this by replacing it with a somewhat similar noun:
The action of seeing the tiger, the man ran away.
This doesn't work. (And don't add prepositions before the gerund to make it work: that's cheating!)
Tiger-aware, the man ran away.
As you see, it works if you replace it with an adjective, so it is a participle.