1. (noun) vertical space available to allow easy passage under something

• headroom
• headway

1. (noun) the distance by which one thing clears another; the space between them

• interval
• separation

2. (noun) vertical space available to allow easy passage under something

• elbow room
• room
• way

3. (noun) permission to proceed

• license
• permission
• permit

1. (noun) the distance by which one thing clears another; the space between them

• clear
• top

2. (noun) permission to proceed

• authorise
• authorize
• clear
• pass