Toxic
Poisonous
noun, な adjective, の adjective
Things that have poison are toxic.
有毒 is a relatively formal word generally used to describe toxic materials or substances. It can also be used for formal classifications like 有毒植物 (poisonous plants) or 有毒動物 (poisonous and venomous animals).
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
ガラガラヘビって有毒なんですか？
Are rattle snakes venomous?
これ、タバコより有毒なんだよ。知らないの？
This is more toxic than tobacco, you know? Don't you know?
どうやら有毒なガスが発生しているようです。
It seems that toxic gas is being emitted.