Anyway ubuntu have dictionary by default. You can find the dictionary from
Application –> office –> Dictionary.
You can find the meaning of a English word using this. When you search for meaning of a word its communicate with online dictionary server for ex. dict.org then get the result. It requires Internet connection. If you not having the Net connection / poor connection its useless. In this situation offline dictionary will helpful to you. Here is how to do this.
Open the terminal (Application -> Accessories -> Terminal) and run the following commands:
apt-get install dictd
apt-get install dict-gcide
apt-get install dict-moby-thesaurus