1close; intimate
2to strengthen (ties, cooperation etc)
3closely; intently; carefully
4(math.) to osculate
Example
wǒ men我们mì qiè密切dì地guān zhù关注zhè这jiā家gōng chǎng工厂jǐ几gè个xīng qī星期liǎo了.
我们密切地关注这家工厂几个星期了.
We observed this plant closely for a few weeks.