In part, or to some degree, but not completely.
"The project was delayed partly because of bad weather and partly due to budget cuts."
In plain English: Partly means that something is only true for some reasons or to a certain degree, not completely.
"It is partly cloudy today, so you might want to bring an umbrella just in case."
Usage: Use "partly" to indicate that something is true or caused by a specific factor to some extent, but not entirely. It correctly modifies verbs, adjectives, or other adverbs to show a partial degree of an action or state.
Partly is formed by adding the suffix -ly to part, creating an adverb that means "in part." This straightforward construction has maintained its original meaning since it entered common usage.