The term fiat derives from the Latin fiat (“let it be done”) and is used in the sense of an order, decree or resolution. Examples include the US Dollar, the Euro, the Chinese Yuan, the British Sterling and the Japanese Yen.
Generally means a type of software upgrade which is done in such a way that it can be backward-compatible (soft fork) or cannot be backward-compatible (hard fork). A fork refers to a split in the network because of conflicts in the following rules resulting in two networks. Forks can be either accidental or intentional.
Fungible means that a specific good is identical among its various units. An example of a fungible token is bitcoin. One bitcoin is and will always be one bitcoin, identical to others in the network. Some other tokens, however, can be non-fungible, meaning that these tokens can have unique characteristics. These tokens are non-fungible, as in they have slightly different properties.