Definition from Wiktionary, the free dictionary
gerund (plural gerunds)
- (grammar) A verbal form that functions as a verbal noun. (In English, a gerund has the same spelling as a present participle, but functions differently.)
- In the phrase ‘Walking is good exercise.’, walking is a gerund.
- (grammar) In some languages such as Italian or Russian, a verbal form similar to a present participle, but functioning as an adverb. These words are sometimes referred to as conjunctive participles.
- In the Russian 'Нельзя переходить улицу, читая газету.’ (One shouldn’t cross a street while reading a newspaper.), читая ‘while reading’ is a gerund.
Derived terms 
verb form functioning as a verbal noun
verb form functioning as an adverb