How to terminate a window in tmux? Like the Ctrlak shortcut in screen with Ctrla being the prefix.
Kent's response fully answered your question, however if you are looking to change tmux's configuration to be similar to GNU Screen, here's a tmux.conf that I've used to accomplish this:
# Prefix key
set -g prefix C-a
unbind C-b
bind C-a send-prefix
# Keys
bind k confirm kill-window
bind K confirm kill-server
bind % split-window -h
bind : split-window -v
bind < resize-pane -L 1
bind > resize-pane -R 1
bind - resize-pane -D 1
bind + resize-pane -U 1
bind . command-prompt
bind a last-window
bind space command-prompt -p index "select-window"
bind r source-file ~/.tmux.conf
# Options
set -g bell-action none
set -g set-titles on
set -g set-titles-string "tmux (#I:#W)"
set -g base-index 1
set -g status-left ""
set -g status-left-attr bold
set -g status-right "tmux"
set -g pane-active-border-bg black
set -g pane-active-border-fg black
set -g default-terminal "screen-256color"
# Window options
setw -g monitor-activity off
setw -g automatic-rename off
# Colors
setw -g window-status-current-fg colour191
set -g status-bg default
set -g status-fg white
set -g message-bg default
set -g message-fg colour191