Its not a word commonly used today but it does describe passion, enthusiasm and fervour towards something.
The Bible encourages us to have spiritual fervour and zeal for God and His way of life.
Its easy to fall in to the trap of working up a passion & to try hard in our own strength to do all the things we think we should do to please God.
In Galatians the Christians had made a similar mistake. Having become Christian by spiritual new birth & life they then tried to live this life without the power of God.
We need to realise that God has given us His Holy Spirit (John 14) to enable us to live His life of Zeal in us.
Thanks Ian (22nd June 2014)